A nice trick for composing lines... put down the bass for a few minutes and try inventing a bass line for yourself by singing it. Yes, singing it. You can do it like "scat" singers do (bow doo dah, bo-do bee dow... bah-dah dee dow). Just form sounds with your mouth, or if you prefer you can use the same sound over and over (dah dah dah, da-dah, da) Sing a melodic bass line that sounds cool to you, and try to forget about 'How am I gonna play THAT'...
Then pick up the bass, and play the line you came up with, using your bass.
I've read about this, and found that it works for me. One really good application... your guitar player (or whatever) gives you tape of his ideas for a new song... while you listen to it, try to come up with your bass lines by singing them.
You can also make up alternate bass lines to songs you hear on the radio... it's good practice. While you're driving, sing the basslines to whatever you're listening to, or sing what YOU would play... how would you do it differently?