Some of them absolutely need to be done at a precise time (e.g. concert tickets), while more often things need to be done tonight, but it doesn't matter what time. Generally I put a reminder in my phone (not a smart phone) but the problem is a have to basically guess at when I'll be available to do the thing.

I'm having the same problem and if I set an "arbitrary" alarm for tasks, I get bombarded with reminders and tend to ignore them automatically.

IMHO the following approach would be better:

• create a list of things you'll have to do until next time you'll be able to see your complete task list, so e. g. until this evening.

So you have everything you'll need to know on this list. Now the only thing you'll need to do is looking on the list at the right time!

This sounds easy, but it maybe is not (my experience).

• One solution might be to create and train a habit to regularly look at your list to see what has to be done.

• Another solution would be to let your phone remind you not about a specific task but only about having a look at that damn list.

Sure, for appointments which really start at a certain time, you can define a specific reminder. But for everything else it should be enough to remember looking on your list, no?

There are applications for smartphones that use the cell location / wi-fi location / GPS to set off an alarm based on your current location. Guess that would solve your problems regarding alarms for home being delayed until you're actually home, or to remind you of getting something at the pharmacy if you either set the location of a pharmacy you know to be on your way, or if you just set it to go off once you're a certain distance from the office.
For Android, go to the Android Market and search for something like "location aware alarm" - which gave me two hits immediately. I also remember seeing something like this for Symbian (though that was a while ago). Not sure that iOS with its limitation of background tasks allows anything like this.

If you have an iPhone, or other iOS device, use the app called Due. You can quickly register tasks, set a time, recurring if necessary and it will remind you when they are due. If you can't do the task at the allotted time then it is simple to put the task off by ten minutes, one hour or one day. Muliple clicks put off the task by multiple minutes, hours, days. If you ignore the alarm, it will remind you every minute or hour until you have done it or put it off. Due is a tireless reminder of tasks.

If you must do a task at a specific time you should plan to be next to the place where you need to do that task by that time, to do it at the right time is a matter of putting an alarm before you need to do the task. This way you should be able to stop with what you are doing and get yourself ready to do the task. Make sure that you don't perform a task that you can't stop at that time, check your alarms if you have multiple and forgot them...

If your alarms collide with other events, you need to use a calendar to schedule your time. That way, you can plan in advance and when your friend tries to schedule the dinner at that time; you say you need to get your concert tickets at evening before they are sold out and hence plan the dinner for the next day...

Tasks that have to be done on a certain day are a matter of scheduling tasks in a task system (like GTD), those that have to be done by a certain day can be given a deadline. Most software solutions provide fields for those and on paper it's a matter of writing the date along. And in the agenda, put it on those days.

If you are in front of computer you can use something like Stickies (free application from Zhornsoftware.co.uk) Or, you can use Google calendar for instance and set reminders with sms notifications. Or you can appoint new tasks using your mobile phone etc... I personally write sms and send it to draft so that I can delete rows in the sms meessage as I get done one by one.

Instead of guessing the time when you'll be available to do it, note all the things that you want to do and then check the list when you get available. Simply put, do it the other way round.

Tasklist is a feature of every cellphone today. It has an inbuilt alarm system which you can use. Use it.

The switching from paper to computer is a common problem.

Currently, I keep everything on the computer, because its easier to edit/rearrange than on paper:

• Tasks and projects ordered in trees
• Active calendar for meetings and appointments (the one I have to use at work).
• Date calendar (people's birthdays, when things expire) this is just a file with a header for each month.
• List of recurring tasks (e.g. cut hair, call parents tidy flat, do accounts)

Every week I:

• Go through the list of recurring tasks, and copy any that need to be done on my to do list/active calendar*.
• Go through the date calendar and see what's coming up, and if I need to do anything about it, it goes on my to do list/active calendar*.

*If something has to happen at a specific date/time, it goes into the active calendar, else its on the to-do list.

I do all of the organising on the computer. When I leave the computer for any period of time, I get a scrap piece of paper and write everything that needs to be done before I next get back onto the computer. When I get back, I cross-off things from the list of tasks, and if there's stuff I wrote on the paper while I was out, they get copied onto the computer-based system. I never have more than one piece of paper.

I find this works well: it's minimal fuss, and I never miss anything, though its amusing to see people at meetings look at my torn scrap of paper and assume I'm completely disorganised :-)

My biggest trick has been google calendar and gmail, I use gmail to remind mysel of thing's to do, and I set goals, appointments, into google calendar.

I also bring my ipod touch with me, so any appointment is entered on that, and when I get home synch's to google calendar.

After years of trying different systems, software and plans, I usually stick to what is simplest and easiest to use, since that's what i'll actually use.