If you have a short haired rabbit, it is a good idea to brush them at least once a week. When they are shedding (they usually shed about every 3 months), more frequent brushing is recommended. During the heavy part of a shed, daily brushing is ideal. Keep in mind that rabbit skin is quite fragile, so be gentle and use a brush designed for rabbits if possible (bristle brushes are preferable; metal toothed slicker may hurt their skin). A fine toothed comb can also be used. Following up with a rubber grooming tool such as a Zoom Groom (see this one at LeithPetwerks, or check the cat section at your pet store) can help clean up loose hair too, or try running a damp (not wet) washcloth over the coat after brushing.
If you have an Angora rabbit, grooming must be a daily ritual. Unless you are showing your long haired rabbit, it is easiest to keep the coat trimmed to a length of about 1 inch or else the coat will be very prone to matting and your rabbit prone to hairballs ("wool block"). You can trim it yourself or get a groomer to do it and just do touch-up trims at home. You must be very careful about trimming hair though since rabbit skin is quite thin and easy to cut accidentally. With these rabbits, daily brushing should become part of the daily routine from a young age (it is a good chance to bond with your bunny, too).
Always be careful about trimming the hair over a rabbit's hocks however, or sores may result.
Removing Matted Hair
If your rabbit does develop mats in its coat, never try to trim them out with scissors as it is very easy to accidentally cut into the skin doing this. Gradually work out the mat by gently separating and combing hair out of the mat a tiny bit at a time, being careful not to pull on the skin. It may take several grooming sessions to work out a mat. Alternatively, you can take your rabbit to a groomer to have the mats trimmed out with electric clippers.
Does My Rabbit Need Baths?
No. Rabbits do not need baths and generally find them very stressful. If absolutely necessary it is better to just do a "spot cleaning" of the area that is dirty rather than subjecting a rabbit to the stress of bathing. If it is absolutely necessary to bathe your rabbit, keep in mind that it takes rabbit fur a long time to dry and it is a good idea to use a blow dryer (on a warm, never hot, setting) to speed the process. Rabbits are prone to overheating, so be cautious. It is best to avoid baths if possible.
Regular nail trims should also be part of the grooming routine. Check the nails once a week when grooming and trim them whenever they get a bit long. I find it is better to do frequent trims even if you are only trimming a sliver off than wait until the nail is quite long and trying to judge how much to remove. Detailed steps to nail trims can be found in "How to Trim Your Rabbit's Nails" and an illustration and more tips can be found in "Nail Trims."
More on Rabbit Care can be found in the Rabbit Care Guide