Rabbits are somewhat naturals at litter training, although some flexibility may be required by the owner. Rabbits naturally pick one or more toilet areas, and owners can take advantage of this in litter training.
First a suitable litter is needed. The rabbit will probably like to lay in the litter box and may even nibble on the litter, so something absorbent and safe is necessary. Rabbit urine also has a strong odor, so something that absorbs odor is preferable. Litters can be found that are made from a variety of materials that work well, including alfalfa, recycled paper, citrus, or oats. Hay can work well too, and some owners even use rabbit pellets as these are economical (but may not be the best choice as the rabbit may overeat if pellets are constantly available). Clay and clumping litters are not a good choice as the rabbit will likely ingest some litter.
For litter pans, cat litter boxes work pretty well, although smaller pans such as cake pans may work for smaller rabbits. If your rabbit tends to back right up to the edge and deposit outside the box, some creativity may be required. A covered cat box is a good option, or a dishpan that has higher sides can work as well (an lower entry can be cut into one side). As mentioned earlier, working with your rabbit is the best way to accomplish reliable training.
Older rabbits are a little easier to train, as they do not need to eliminate as frequently and their natural desire for cleanliness is more developed. However, once rabbits hit puberty the desire to mark territory becomes very strong, so spaying or neutering by 4-6 months of age will make litter training a lot easier. Along with health benefits, undesirable urine spraying and other marking behaviors will be drastically reduced. As well, marking will be reduced if the rabbit feels secure in its home. Tips to reduce the territorial marking by rabbits include not reaching into the cage to pull the rabbit out (makes a rabbit feel threatened), not forcing the rabbit back into its cage, and doing maintenance tasks (cleaning, refilling water, etc.) while the rabbit is out which will not disturb the rabbit while in its cage. It is preferable to allow the rabbit to come and go from the cage at it's own will - which may not be the most convenient but if the rabbit is marking its cage or cage area this may be necessary. Try to set up the cage so the rabbit has easy access in and out, let the rabbit come out on it's own terms and gently try to herd or entice the rabbit bake into it's cage rather than picking it up and forcing it in.
To start, confinement and supervision is the key. If a rabbit is allowed to urinate and defecate wherever it likes from the beginning, it will be much harder to train. Keeping in a cage (not too large at first) with a litter pan, and watching carefully when the rabbit out out will be necessary in the beginning. Place a litter box in the cage, and note where the rabbit goes. It may start using the box, or it may pick another corner of the cage. If this is the case, then move the litter box to here the rabbit seems to prefer. Flexibility on litter box placement may be necessary both in and out of the cage.
Once the rabbit is using the litter pan in the cage, allow the rabbit out of the cage in a limited space. Provide a litter box outside the cage, and perhaps make it enticing by placing a a treat or favorite toy in the box. Watch the rabbit for signs it is about to urinate or defecate (usually backs up and lifts tail slightly), and try to herd it to the box immediately (if your rabbit is very calm about being picked up it should be okay to place it right in the box). If the rabbit uses the box, give the rabbit a treat: food, toy, petting, or praise (whichever would be the best reward for the individual bunny) right away. If you notice the rabbit tends to head to one area to do its business, consider putting the box here.
Accidents will happen, and punishment has no place in training a rabbit. If the rabbit has an accident, it won't hurt to take the rabbit to the box, although the rabbit may not make the connection. Clean the spot with club soda, diluted vinegar, or a commercial pet stain/odor remover, and watch the rabbit more carefully while it is out. The key is to get the rabbit to the box before it goes.
Over time, the rabbit should develop a preference for using the box, and the freedom of the rabbit can be increased. More boxes may be necessary as the amount of space your rabbit is allowed increases, as the rabbit may not be inclined to travel a relatively long way to find a box. Again, if the rabbit repeatedly chooses one place in he room to eliminate, consider moving a litter box there, it is often easiest to work with what the rabbit naturally wants to do. If such a location is really incomvenient, try using a box there for a while and then gradually move it out of the way a bit.
The process sounds daunting, but usually goes pretty smoothly as long as the owner works with the rabbit's natural tendencies and provides undivided attention to the rabbit during it's free time in the beginning. Establishing a routine with your rabbit will also help, as rabbits are creatures of habit. Sometimes a previously trained rabbit will get a little careless, and this usually means starting from the beginning with restricted freedom until the rabbit is trained again.