The way that I do my bars is by doing the same riff but in many different places

G———————————————————————————
D————————2——5———————————————
A———2——5—————2——5——————————repeat as many times as you want, then...
E——3——————————————3————————

G————————————————————————————
D————————2——3————————————————
A————2——5————2——5—————————————And when your done with it.....
E——3——————————————3—————————————

G————————————————————————
D————————————————————————
A————2—3—4—5—4—3—2————————————
E——3————————————————3——————————

The point of this is that you can do this anywhere on the bass and it will sound good. You 
should also look at the guy who did the first tabbing of the 12 bar blues so you can mix and 
match what you want.