Of course, Chris has the best advice!
What has worked for me to improve my accuracy:
1. ALWAYS try to figure out what you did wrong on each solve, as soon as you finish it, if it's a DNF. You may not always succeed, but if you're persistent about it, you should certainly be able to figure out what went wrong most of the time. If necessary, reapply the scramble and work through the solve sighted to see if you can figure out where you went wrong. If you know what you're doing wrong, you can concentrate on it and fix it. I think this is REALLY important if you want to be accurate.
2. Do lots and lots of solves, back to back. Go for averages 10/12. If you get 2 DNFs, just keep going and try for a rolling average. The more solves you do, the more accurate you get. One of the things that leads to lots of my mistakes is memorizing or solving the wrong sticker or piece. The more you do this, the more anchored the piece letters (or whatever you use) become in your mind, and the less likely you are to make those mistakes.
These two things have helped me to get over 50% accurate on 5x5x5 BLD. So it has worked very well for me!
When you analyze your solves, if you find your memorization is the problem, then you can spend a little longer memorizing each piece, to anchor it more firmly, and then speed up once your accuracy is up. If your memorization is not the problem, I don't think it helps all that much to slow down very much. A little more care on each algorithm can help, but honestly that's only going to cost 10 seconds or so. There's no reason to solve really slow - if you do, it's not practicing for solving the way you want to be able to solve.