I often call Umbra my stinker, and usually it's just a term of endearment, but sometimes she truly is quite smelly. This time I realized it was time for some spot cleaning when Umbra kept sticking her butt in my face while I was sleeping.
For the most part, rabbits do well enough on there own when it comes to bathing, and you should never give your bun a bath unless recommended by your vet (generally elderly or handicapped buns may need a bath), however there is one area that is often the source of foul odors. If you have a stinky bunny its probably time to clean the genital scent glands. Although not a very pleasant chore (for human or lagomorph) it is very simple. All you need is a damp cloth and/or cotton swab (dry or wet). While holding your rabbit, locate the scent glads on either side of the genitals (they are just in the folds of furless skin). There may be some dry build up that can be brushed away with the dry cotton swab, then gently wipe the offending areas with the damp cloth.
WARNING: the smell will get worse once you locate its source. It maybe be advisable to hold your breath otherwise gagging and nausea may effect the human.