One great way to do this is to go on what I call a "sampling bender". I'll set aside a day on the weekend and spend that entire day doing nothing but sampling new sounds. If you don't have anything interesting laying around the house to sample, no worries. Scour your CD collection/music library for isolated instruments and sounds that you can easily snag. Listen to just the sound portion of your favorite movies on DVD and see if you find anything that might be interesting.
Snagging the samples is just the first step, though. The real fun comes into customizing the samples you take and turning them into entirely new sounds. Try layering 3 different snare drums with different qualities, EQ them individually, compress them together, and bounce it out again to create a new, custom sample. Look for unusual ways you can use ordinary sound. The squeak of a chair's legs across a wooden floor from your favorite scene in a movie might sound wicked transposed down two octaves and fed through a big reverb. The gasp of the shocked heroine when she discovers the dead body might make an interesting attack transient when layered over another synth sound. Loop some breathing underneath a pad sound for some interesting added texture. You get the idea.
I think you'll find these marathon sampling sessions really productive if you give them a chance, mostly because they're a lot of fun. And as an added benefit, you end up with a nifty new sample library!