Ironically the best way to build up speed is to play slowly and build up the tempo gradually. Play something using a metronome at a comfortable tempo. Then increase the BPM by say ten. When you have nailed that, increase by another ten etc. If you cant play cleanly and comfortably at a given BPM, return to a lower one. After that, it is a case of time, patience and lots of practice.
If you want a good work out for the L/H, get "Bass Aerobics" by Jon Liebman. http://www.amazon.com/Bass-Aerobics-.../dp/1423495632
I am using it at present and IMO it helps improve the L/H technique. The tracks on the accompanying CD are fairly fast. I usually learn the track first, then use a metronome at a slower tempo and increase it gradually until I can play along with the CD. If you have soft ware to slow down the CD that would be ideal too.