OK,,,,I think to be a good sampler, u need to have a very objective and open ear. Dont limit yourself to what u think needs to work for a specific track, instead sample anything that u find interesting and find a home for it later....As far as making a sample fit a beat, thats all about practice and something that comes with time i think....U just need to keep trying things till they fit and feel natural.
If anything, once u identify what sound u want to put into a beat, the next step is to know what meter your working in....If its 4/4, which is most common, then count out loud to your sample in a 4 count, like 1, 2, 3, 4....and then over again in a uniform fashion, if the sample doesnt seem to even remotely fit the count keep tryin other potential sounds.
888888888888888888888888888888888888888888888888888888888888888888888
This of course is pertaining to finding a sample that would work well as a 1-4 bar loop....If your just searching for sounds that are going to be used sort of in the same sense as a synth, then anything goes basically....Just try to pick sound parts that are not entirely cluttered with percussive elements, that usually makes everthing easier......
Good luck!
STEEZ