I'm having trouble keeping my center caps on. Any ideas on how to get them on correctly or are there better aftermarket ones?

which ones do you have? year/ model?

The ones for the 88-00 C/K pick ups are notorious for losing the black cap screws that hold the covers on. the Silverado style 99- present they are locked into the cover unless someone used an air gun to loosen them.

check and make sure your threads in the plastic caps are not stripped (either style).

also as far as after market ones I have not seen any that are even noteworthy