Actually doing things
This is what it's all about. Everything else exists just to support this bit: actually doing the things you need to do.
Doing is usually easy when it's clear what you need to do (a well-defined next action), and you have all the things you need for doing it (proper context), and the motivation to do it. If you've got your GTD system working smoothly, but things still don't get done, the problem is usually motivation, or at least that's the big problem for me.
Sometimes the problem is that the next action is defined vaguely: it's not actually clear what you need to do. Perhaps it was clear when you were planning it, but you didn't write down enough details to remember later why you need to do it, or exactly what needs doing. "Call Clara's cell" may be what you wrote down, but you can't remember which Clara, and whether to call her mobile phone or the jail? You need to describe next actions with enough information that you don't have uncertainty.
Sometimes it's because the thing to do is unpleasant, or boring. I have no good solution for that, except to grow up. (I'll be doing that any decade now.) I myself have a habit of skipping over next actions that I don't particularly enjoy, with the result that they may hang about for months in my next actions list. I'm also very gullible so when I tell myself that they've not been there for very long, I believe myself. That's why I put a date (at least year and month) on every next action, so I don't believe my own lies.
Update: One thing that seems to work for me is to find the tiniest little part of the unpleasant thing that needs doing, and do that. Repeating that a few times is usually enough to break the blockage. For example, if the unpleasant thing is "wash the toilet bowl", the tiniest few things might be "locate the toilet brush", "locate the disinfectant", and "find rubber gloves".
After you've done something
When you're finished with a next action, you can delete it from your list, or cross it over, or otherwise mark it as done. This can be a very satisfying feeling.
Instead of deleting, you may also want to move the item from your next actions list to a list of finished stuff. Such a log can also be quite satisfactory to read, later on. I've found that deleting things gives me more pleasure, though, and I keep track of what I've done using a journal instead.
I also write summary entries in my journal of things that I've done or that have happened, as part of my weekly review.