The good trick I learn about self-motivation is:
Avoid situations when you have to motivate yourself

my way to deal with that is to start writing my ideas about tasks I have to do into simple paper notebook - not organized in any way - just by giving each idea a number.

Every time when I have to motivate myself I'm just taking notebook and start writing my new plans / tasks and reading old ones.

It helping brain to pick up ideas/solutions , and when I'm ready to start on my coolest task , I'm just starting immediately.

The important thing I try to remember is just to pay attention. If you're blindly walking around you're going to run into everything. But if you're watching where you're going, it's a lot easier to avoid roadblocks.

Even with mindfullness, though, it can be hard to refrain. Try meditiation and/or martial arts. Those will improve your self-discipline and a whole lot more!

You mention focus. There are studies that say there are two different mental processes that form what we call attention. One is like focus (zooming in on the task at hand) and the other is like ignoring (filtering out the inner chatter and outside noise). So it would be helpful to consider them separately - not as two sides of the same coin so to speak.

So self discipline could be described as first engaging in a task and then paying attention to that task.

Engaging in a task usually requires you to prioritize that task. It helps to visualize the end goal, the feeling state that will result from your completion. Like with forming a new habit, it can be helpful if you create a reward for completion and even for engagement.

Focusing on a task means increasing attention and reducing distraction.

To reduce distraction you can de-clutter the work environment. Soothing music, limited distractions, no phone, no emails, just the tools you need and a clean workspace are all helpful.

You can also de-clutter your mind. Think about all the things that are near time in your mind, that next chore, dinner, picking up the kids, that pile of laundry. You can put them all in a list and prioritize them. Commit to them being brought back into your attention when the time comes and no sooner. If any of these tasks continually distract you, notice that. Tell your mind thanks for worrying but you will get to those tasks when you said you would.

A mindfulness practice can help with this internal chatter. If your mind wanders you can begin to notice it and go back to attending to the task at hand. Your mind might also go to some other task which you can then just put on the to do list and resume your given task.

This mindfulness practice can also help you become aware of your resistance to getting things done. Someone already mentioned Pressfield's The War of Art. While not scientific, his principles are useful. Just start by taking note of the things you do to distract yourself from the task(s). Like all change, awareness of the current state you want to change from is important.

And to develop self discipline over time, tackle the smallest things first to develop more confidence. Don't skip the rewards either.

"What I am trying to achieve is be able to work a specific number of hours during the day. I work from home as a free lancer, I have good days, bad days, good weeks and bad weeks but I'm not able to get myself to work in a consistent manner. Lets say 8 hours each day."

I think this is called life. It goes up and down and isn't consistent at all. The question maybe to ask is: "Where is my focus and why do you think you lack self-discipline?" The answer will clear up your real motivation. You can keep scratching, but where is your itch coming from?

Henepola Gunaratana sums it up very well..

"Discipline" is a difficult word for most of us. It conjures up images of somebody standing over you with a stick, telling you that you're wrong. But self-discipline is different. It's the skill of seeing through the hollow shouting of your own impulses and piercing their secret. They have no power over you. It's all a show, a deception. Your urges scream and bluster at you; they cajole; they coax; they threaten; but they really carry no stick at all. You give in out of habit. You give in because you never really bother to look beyond the threat. It is all empty back there. There is only one way to learn this lesson, though. The words on this page won't do it. But look within and watch the stuff coming up-restlessness, anxiety, impatience, pain-just watch it come up and don't get involved. Much to your surprise, it will simply go away. It rises, it passes away. As simple as that. There is another word for self-discipline. It is patience." -Henepola Gunaratana