This thread should help you a lot- lots of tips etc in there...
Otherwise, I use a cotton bud/Q-Tip? Soaked in baby oil, or you can use warmish water, and use it to gently ease out the gunky stuff. I usually make a bunny burrito, and hold them very still. You have to be really careful as the skin round there is really delicate, but the baby oil works wonders as it doesn't 'drag' on the delicate skin at all...
Good luck! I did all my 4 bunnies today and although I got through them all in about half an hour, it was a much stinkier job than I remember lol! :shock: