The trick to getting a good sample library is to sample everything you're allowed to or can get away with. It doesn't matter if you like the sound or not, eventually you might have a use for it.
Rip sounds that make transitional effects, ambience or individual note plucks of instruments. Don't rip riffs. Rip sounds from DVDs, other music, sounds you hear of the net, the WAV files you find in computer games, free samples off music mag covers, sounds from internet libraries. It's a bit naughty, but so long as it's not distinctive and nobody could recognise it as their sound, nab it!
With your sample library, use your sample editor to clean up, trim and crop, edit, pitch and time stretch your samples so they're prestine and exactly what you want. Use Cool Edit or something. Above all though, sample everything you can and work to creating a really well indexed directory full of every sound you record.