Probably every band in the known universe (and maybe even some on alien worlds!) have record 12 bar blues tracks, so there's plentty of examples out there.
Two I learnt rencently for a gig were 'Pride N Joy' and 'Texas Flood' buy Stevie Ray Vaughn. The former mentioned here makes good use of the Major6th scale tone in the bassline - as per Stephanie's post. The two are very straight examples of a twelve bar blues.
The key thing in twelve bar blues is the turnaround. The V chord, to the IV and back to the I chord. Using Stephanie's example in G the D7 to, C7 back to the beginning again, G7.
Another note worthy thing is that (in many many many cases) all the chords in the progression are dominant 7 chords, which means there's only one scale you need learn to get through the vast majority of rock-style blues jams, mixoldian.
Best way to learn teh blues is to listen to loads of blues-based records, jam blues with mates as often as possible, drop out of school or work, become an alcholic and shoot your dog...
