I've tried reading some of the books in this, but they all seem very complex!
Which books have you tried, Laura?
I'm self-employed with a number of clients, some of whom I have several projects running at a time. I sometimes find myself hunting around to make sure I've completed the projects in good time. That's why I've been trying to get to grips with David Allen's Getting Things Done for the past four or five years. It might seems complex to begin with, but it is a system that works - and you get to keep your working files close by. He has a nice work flow that you can follow.
There are several things you have to do with any system: capture everything you have to do, decide what to do, do it, review what you have to do regularly so nothing (or very little) slips through the cracks.
Ultimately its down to discipline - remember to use the system (my biggest downfall is just jumping in to do whatever is in front of me). It'll need time to get it working, and time to maintain it. But the time you invest in those aspects will help in the long run to give you better choice over how you use what's left of the day!
David Allen's new book - Making it All Work - is out. If you have read any of his you could start with this one as it covers what is in the others. Also worth visiting his web site if you are interested.
Whatever system you choose, try to stick with it for as long as possible. I've spent years hunting for the elusive perfect system. I've yet to find it, and doubt I ever will, but what I'm using will get easier and better with the passing of time. Good luck!