Two options, cheap and cheerful... http://store.rc4wd.com/Rocker-Dig-II...er_p_1321.html

I've used it, it's 'ok'. The drag brake isn't the strongest ever, but its strong enough for most.

And the pro option... http://www.punkrc.com/index.php?main...products_id=28 - I haven't used it, but serious pros swear by it, so I have no doubt its an excellent product that does exactly what it says on the tin.