My algorithms have to have no rotations, since I find those algorithms much slower. (I have used 2 or 3 algorithms with 1 rotation)
They must be 3-gen. (If an algorithm isn't 3-gen, I find it harder to perform.)
I then prefer algorithms that are only 3-4 QTM above the optimal solution.
R,U,F algorithms that fit this list, are my first choice, then R,U,L and R,U,D are my second choice, then R,U,B are my last choice. (This only applies if there are multiple algorithms in the database that fit all the above "rules")
This is usually how I find algorithms that I might use; but then once I have found a few that fit the list, it comes down to what I think feels better.
Edit:
This works good for OLLs, but with PLLs, a lot of fast cubers have already made videos of what PLLs they use. Usually they use some of the best PLLs out there.
If you haven't learned all your PLLs yet, here are some videos that might help you out.
http://www.youtube.com/watch?v=LW4TYyHH-1s&feature=channel_video_title (Erik Akkersdijk)
http://www.youtube.com/watch?v=fWyjLAiEL9o (Breandan Vallence)
http://www.youtube.com/watch?v=kossdjLYqsc (Feliks Zemdegs)