I have experimented with this some and came up with something I think works well...
3M makes a "dual lock" type of velcro (it's not really velcro but works ths same way). Both sides are hard plastic, with adhesive tape used to mount the pieces. With this type of set-up, you can put a piece on the motor and then as many as you want on the board...if you want to try running the motors in different spots.
It definitely works a lot better than velcro (where one side of the attachment is softer and the motor tends to vibrate out of a tight lock).
I bought mine at K-mart...but this ebay auction is actually cheaper because you get more for your money.
http://cgi.ebay.com/3M-BLACK-DUAL-LO...photohost ing