Starting things is hard to do. So when we do start something it feels good. As humans we like things that make us feel good. The problem is, when the task is difficult, that good feeling only lasts for so long. So even if the task isn't finished, we want that good feeling back. Let's start something else. Repeat the process. Eventually you've started a bunch of stuff and you've finished none of it. The thing is finishing something will multiply that initial good feeling. The reason being because you overcome more. When the good feeling wears off, you're faced with choices: start something new (to get the good feeling back) or push through the (potentially difficult) task to the finish (prolong feeling good). If you can push through you're potentially compounding the "feel good".
In order to do this you have to focus. More specifically you have to F.O.C.U.S:
Follow
One
Course
Until
Successful
I didn't come up with this, a friend posted it on Facebook. From this point on though, I'm going to pretend it's mine (that's how stealing works). Now this sounds good and makes a good acronym but the question is how do you stay focused.
I've got a few suggestions. First off keep the end in mind. Next anticipate the potential roadblocks. Finally break the task up into smaller tasks. By keeping the end in mind it's actually a reminder of why you started in the first place. Thinking of the end keeps the benefits of finishing fresh in your mind.
Thinking of the end also helps you to think of the potential roadblocks you may encounter. If/when you think of roadblocks ahead of time, you can prepare for them. If you're prepared for them then you can possibly avoid them all together. This happens because you become attuned to the signs that a roadblock is ahead. When you're prepared for roadblocks, or you can avoid them all together, your progress towards the end goal doesn't slow as much. So you won't be tempted to change toward a different task.
Think of potential roadblocks will help with the last suggestion also. Roadblocks make for good break points. Getting to and through these points will help keep you motivated to get to the next one. Another way you can break a task into chunks is to break out the steps. What I mean is, if there a particular order to complete your task then simply write out the steps, like an instruction manual or recipe. Then follow the steps.
These are a just a few suggestions. I'm sure there are tons more out there. Actually if you can think of or know of any let me know in the comments. The more the merrier. The point here is that by concentrating on finishing one thing you actually get more accomplished in the long run. Also as much of an accomplishment as it is to start something, it is a much bigger accomplishment to finish something. So stay FOCUSed.