Hi dude,
I know I posted this before and you've probably seen it - but these videos do illustrate working up thumb strokes and finger plucks on each string. Also try and mix in the whole open-hammer-pluck thing - try learning a bass groove - along the line of 'Me and my bass guitar' - that bassline uses all four strings and obviously grooves like a mutha
Try and get past just the technique and start trying to groove with it - I know it's not easy but also try working on arpeggios - they obviously (normally) involve using more than one string - I play a couple on the second video here:
http://www.munkio.com/music/music_lessons.html
I hope that helps a bit
M