Impact drivers and shredded screw heads
I'll assume a few things and you can correct me if I'm mistaken. I'm guessing you're using phillips screws (not pozi-drive, nothing fancy like torx). I'm also assuming you've got a good quality driver bit, that hasn't been used too long. 

So, a few suggestions:


make sure you're screwing directly in line with the screw. Any angle variance is bad.
get behind the impact driver and push. You need to maintain a good amount of pressure.
buy high-quality screws... I've had particularly bad results from big box gold screws. The exterior grade ones (gray paint) seem much better. Most of the name-brand ones (as opposed to the 'house brands') are better. 
also consider alternate driver heads -- torx, robertson, pozi-drive, unidrive. (The alternate heads frequently go hand-in-hand with better products, as well.)
if your bit is worn out from a few too many off-angle screeching spins, throw it out.