I've seen a few people use thimbles and this guitar pick that wraps around the finger. But depending on what sound you are looking for you can use anything. A beer bottle would work.
ive rocked out with drumming brushes, there cool cuz theyhave the brush part, and the hard part on the other end, plus you can pull the bushes out as far as you want (to an extent) and then change the sound even more
i've tried many different ideas and drum sticks seem to suit me best, since my washboard is kinda set up like a drum set anyways. i also use the drum sticks to bang aroun don various trash item i find aswell...