Why do my drywall seams keep cracking?

I do a lot of repairs and drywall work, but not really enough to get the expertise to make these seams right. I use tape, I have to sanding down, but down the road they always crack. Could I be painting too quickly? I have used different types of mud, I have never used any quick setting muds though. Just normal joint compound, What am I doing that is keeping me in amateur leagues?