This is how I strum the bass:
1) Hold your right (or plucking) hand in a fist, pointing towards the ground.
2) Quickly flick your fingers out and downwards, contacting the desired strings with the fingernail tips of your middle and ring fingers.
3) You can strum again on the upstroke by drawing your fingers back up into the "fist" position, which conveniently leaves your hand in the perfect position for the next downwards strum!
A few players, like Stanley Clarke (check out the first two-and-a-half minutes of 'School Days') and Mark King use this technnique.
If you want to see this technique in action, go to
http://www.forevernow.com (a Level 42 fan website) and find the video clip called 'Bass Solo Copenhagen' - it shows it quite well.
Good luck, and let me know what you think of that video clip.
David