Sampling techniques and improving the sound of a sample.

When sampling, and trying to bring out a particular sound (lets say baseline) what is the best way of doing this? Is it all eq, or does one mimic the base-line by adding there own midi notes with the same tone as the original?

Also when I use stretch a sample to a beat that has existing drums I will find that the drums conflict with the drums I put on top of it (hopefully i'm making sense). Is there a good technique to mask these drums?