i put my ipod on shuffle...and listen for songs that have good bass lines, then learn them. ive been teaching myself for a few months and so far, its working out pretty darn well. as long as you have a variety of music, you shouldnt get bored with it.
RATM has some good stuff, its pretty easy and kinda repetitive in the songs, but the sound is cool. i like know your enemy and bombtrack--love the beginning of that song.