• Email is very distracting if you are reactive to it. Set a schedule and stick to it: e.g. check emails not more than once an hour; determine the priority of replies/actions to be taken based on new email and only perform the critical ones right away, otherwise "park" them and get back to the task you were doing.
• Make sure you have filters set up in your email to ensure that anything not so important does not get in the way, e.g. use Priority Inbox in Gmail and set filters for newsletters etc so these messages skip the Inbox. Then only check the rest of the email (which has been filtered) at set times (like start and/or end of the day).
• Use Busy and/or Invisible in your IM. Only reply to important messages and again "park" not so important ones (this is easy with Google Talk/Gmail because you can come back to the chat later).
• Separate work and personal IM to eliminate friendly distraction.
• RescueTime is great but don't get obsessed with it. Again, check it only on a regular schedule, i.e. at the start/end of the day/week.

Overall, the best thing you can do is stop being reactive to distractions. Acknowledge, park and come back later when you have time (alloted for this kind of catch up work).

Bonus: If you really want to make your Gmail inbox to be as un-interruptive as possible then set all labels (expect for vitally urgent) to be hidden so you are not bothered by unread counts of other less important labels.

P.S.: I think it's almost a must that you clear out the Inbox to begin with and don't have any unread email in it so you are not tempted to read unread emails from the past. Once you have filters, etc then unread email of lesser importance will be safely hidden from you until the time you can get to it.

What works well for me is the Pomodoro method, which forces you to focus for 25 mins, see: http://www.pomodorotechnique.com/

This is a major issue for me as well. I'm still trying to improve my concentration because I will get distracted by anything, but here's what I do so far:

Music - I find when I'm listening to music I'm much more productive. It helps that it blocks out coworkers and background noise, but just in general I'm more likely to focus on my work rather than goof off. I think I'm different in that I can listen to almost any music and still concentrate so if this is hard for you, try classical or other instrumental music.

Leechblock - This is the only "internet blocker" program that I've found that works for me. Nothing for Chrome works, unfortunately, because it's too easy to hit shift-esc, bring up the (Chrome) task manager, and kill them. So I have to use Firefox, and Leechblock is just hard enough to get around that it usually works for me.

Pomodoro - I use a simple variant of the Pomodoro method - 50 minutes on, 10 minutes off, and that's it. Basically Leechblock only allows me to access anything non-work related for the first 10 minutes of every hour. I find this is better than 25 minutes on, 5 minutes off, because say I need to write a personal email - I may not be able to finish that in 5 minutes.

More details on my Firefox setup: I have two profiles - one personal one, one work one. The bookmarks on the personal profile are synced with those at home with xmarks. Leechblock on this profile is what I described above - everything is blocked (with very few non-distracting exceptions, like Pandora for example) at all times except the first 10 minutes of each our. The work profile only has work-related bookmarks. Leechblock on this profile blocks everything not work-related at all times.

So when I want to go to facebook, gmail, etc. - I have to use the personal profile, and it has to be in the first 10 minutes of an hour. Yes, I can open Firefox's profile manager, create a new profile, and use that, but doing that, combined with the fact that I have to log into all of these sites (e.g. gmail) over again make it just cumbersome enough that it usually stops me from doing it. But if I really need to quickly send a personal email, then I can do it.

On a Mac or unix system you can block time-sucking websites at the system level by adding

127.0.0.1 www.nytimes.com
127.0.0.1 www.reddit.com
127.0.0.1 www.otherdistractingsites.com

to your /etc/hosts file. This tells your computer that the website is located on your computer. Since it's not, your browser will fail to load anything if you try to navigate to that page. A few sites are more complicated to block using this method. For instance, to effectively block facebook I had to add this to my /etc/hosts:

127.0.0.1 www.facebook.com
127.0.0.1 static.ak.fbcdn.net
127.0.0.1 www.static.ak.fbcdn.net
127.0.0.1 login.facebook.com
127.0.0.1 www.login.facebook.com
127.0.0.1 fbcdn.net
127.0.0.1 www.fbcdn.net
127.0.0.1 fbcdn.com
127.0.0.1 www.fbcdn.com
127.0.0.1 static.ak.connect.facebook.com
127.0.0.1 www.static.ak.connect.facebook.com

To unblock any of these urls, you have to reopen the file and comment them out with a # character. I prefer this method to browser plugins because it is so easy to circumvent a plug-in by simply opening up a different browser.

For writing or coding, do all your main editing and composing in a full screen text editor. Even if your final formatting will be done in a WYSIWYG editor like Word or an email client, this spartan computing environment is all you really need for most tasks. I spend ~70% of my tube-time with just TextMate and Terminal because they are all I need to get things done.

Update: On windows you have the folder in "C:\Windows\System32\drivers\etc" even if it can be hidden. You might need to run your editor as administrator to to edit it.

Remember that, at least on windows, mobile.mysite.com and mysite.com are considered different hosts. This means that you might need to block all sites with and without the www.. Or that you might need to add international.nytimes.com, technology.nytimes.com, etc...

Saving this folder sometimes takes time until you close all browsers. It might be a good thing as this will prevent you from swithching off and on :)

I pay attention to the following:

Physical Work Environment

I make sure to have a quality drop-down keyboard, a good wireless mouse and a quality chair. If I'm physically uncomfortable I'm not going to be able to focus well. A/C or heat should be comfortable.

Multiple Browser Windows

A technique I developed without being aware of it: Not browser windows or tabs but actual different browsers, such as Chrome, Firefox, Opera, IE, etc. I use Opera for personal stuff, personal gemail, bookmarks, logins, etc. I use Firefox for work, work gmail, work bookmarks, work logins, etc.

Multiple Terminal Windows

Similarly to browser windows, for terminal windows I use split panes to keep stuff in view and in focus rather than requiring an action to reveal it as in tabs.

Multiple Monitors

I find multiple monitors useful and tend to group activities on them, e.g. email and IM on one and development on the others. This way I can glance at communication without losing focus.

Visual Blockers

I am easily distracted in open office environments and so I find a combination of multiple monitors, desk locations, bit of cardboard and other techniques I can block out the view of others.

Audio Blockers

Over the ear headphones work best for me, plus they give the signal of (both ears covered) very busy, (one ear covered) busy but interruptable, (not on), available for all questions.

Cell phone

Mute it or turn it off. Turn off unneeded notifications.

Network

I like a high bandwidth wired connection so I am not being distracted by a slow connection.

Email.

Manage email with filters

IM

Be selective in the channels you use and the conversations you stay in. Go into quiet mode when you need focus.

Those are things that very helpful for me:
1.pomodoro technique and pomodoro timer
2."cans" (big headphones) and music
3.planning

1. Analyze what waste your time
2. Keep disconnecting from Internet. Whether you are reading online journals, news, study materials, or anything else, download their offline copy. And read from them.
3. Make a rule to be away from social sites like facebook, google+, twitter. Access them once in a week or on weekend only.
4. Use Google calendar like utility which can remind you for an event. It can be synced with facebook events and can update you by sending SMS. No need to check facebook daily yo know about any event.
5. Filter mails in gmail. I heard that you can set SMS reminder for mails. Set it for important mails. So you'll not have to sign in.

Out of computer: 1. Keep your mobile away. Or keep it on silent (excludes important number like your boss, or parents)

Remember: working longer on computer affect your eyes, health, and your productivity as well. So take rest of 10-15 mins after every 1-2 hours.

Try the following:

1. Virtual Desktops - like Deskspace that comes from Otaku Software. Allows you to get what Linux always had - multiple desktops.

I use it to move mail clients, messengers, browsers etc. to different virtual windows. If you don't see it, you don't get interrupted

1. Use full-screen editors like OMMWriter. Comes with its own "easy listening" sound bytes and essentially removes distractions. LifeHacker has carried many software examples that let you get a full screen editor

Maybe this not your answer but I want to say my experience and maybe help someone:

I think check email , facebook, these web sites and etc are not the main problem. The problem is what are you doing at that time which you neglect your work and do to surf the web. I think we should think about that in this way.

I think about my self and my work and also my useless web surfing. I got this result:

During work when I suppose to those parts which are not routine. I go to facebook or check email and etc. And one way of staing focus is : determining the challenges part before stating your daily task. and try to find them before sitting in frond of your PC.

First of all: clean up. I clean my desk, desktop and web-browser as well (I close facebook, email, twitter and other distracting tabs). The pomodoro technique and a paper to-do list are very helpful. Additionally, I convince myself that the work I am doing is valuable and interesting as well as I set myself a reward for completed job/working day. I believe that the right motivation is our immunization against distractions and helps us to achieve higher productivity.

There has been some research done that says that getting to 20 minutes focused on a single task/job is about the amount of time you need to really get focused and effective. So going between tasks every few minutes is super non-productive. Ever since I read that I've been very aware of the "20 minute rule" when starting a project and once I get going it's very easy to stay focused, knowing I have at least 20 minutes committed to whatever I'm doing. And of course, going longer becomes much easier once you put in 20.