Hi maxn, I believe the subject of permutation and combination is not well covered, or just not at all, in books such as Kaplan
or PR. I used them myself and was very disappointed when it came to that matter although PR2005 CD has some good combination questions if you reach the high 40Q scaled score.
The best resource you can have, and this is not because I am biased, is certainly gmatclub. Just do a topic search in the math forum
and type in probability, combination or permutation and you will get a host of problems. Work through every single problems and you will eventually understand the mechanics of it. Ask questions if you are not sure.
To be honest with you, combination was a topic that was not covered during my high school time and I absolutely started from scratch. I learned everything I needed to from this website and I even ended up writing up some combination problems myself with some old members. I therefore suggest you to painstakingly go over those problems here and you will certainly get better in that respect.