I've recently formed a band with a group of friends, playing mainly blues (12 bar etc.). The song we have done recently are Fleetwood Mac's "long Grey Mare" and Canned Heat's "On the road again". As you will know the basslines are simple, I can play way harder lines than this and I would like to stick a couple of bass sool's in but I'm not skilled enough to make them up. Any help from an experienced bassist would be very helpful (on either song). Can anyone help me.

learn as many scales as possible, and improv in 'em.
if you're doing bluesy stuff than the blues and dorian scales are a given. Once you've got that down, then just mess around in 'em until you come up with something cool...