One of my favorite benefits of following a GTD workflow is that it eliminates a lot of the decision making for you. When it's time to get work done, just fire up your task manager of choice, switch to your list of available next actions, and pick one. Having defined, physical next actions for each of your projects is the key to moving them forward. But sometimes you can get stuck and lose momentum. You may forget why a project is important.
I've found that this can happen for long running projects or for projects that aren't clearly defined with next actions. For the latter, the solution is simple. Move your focus all the way down your hierarchy of tasks and come up with the very next physical thing you can do to move the project ahead. No matter how small that action might be, it will count as forward progress if you do it. And that might just be enough to get you going again.
But for projects and tasks that have been on your mind for seemingly ever, or for those that you just don't remember why you signed up for them in the first place, it can be helpful to go in the opposite direction.
Take a look at the task you're procrastinating on and move up a level to its parent. Do you remember why you added that to your list? If it's a project, are you still committed to doing it? If you're not sure, go up another level. Is it clear why that is important to you?
You can repeat this process all the way up to your areas of focus. Is it your career? Your side business? Your family? Whichever area you land on, it should be an important tent-pole in your life. You should be able to make the connection between why it is important to you and how that one small action can move you closer to your goal. And, hopefully, that'll be the motivation you need to get un-stuck and moving forward again.