First a suitable litter is needed. You 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 ideal. Do not use clay or clumping litters, or cedar or pine wood shavings. Organic or paper-based pellets and litters are a good choice (brands include Critter Country, Eco-Straw Pellets, Gentle Touch, Cell-Sorb Plus and Yesterday's News - see Top Alternatives to Cedar and Pine Shavings for more options) Some owners simply use rabbit pellets as litter. These are economical and safe, but are not a good choice if your rabbit continually eats extra pellets from the litter box and/or is overweight.
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). The larger size of corner litter boxes might work well for smaller rabbits too, as these usually have fairly high backs.
If our rabbit tends to tip the pan or kick the litter out, try a heavier litter.
Steps to Litter Training
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. At first, keep your rabbit primairly in his (or her cage), which should be fairly small at first, with a litter pan. Place a litter box in the cage, and note where you rabbit eliminates. He (she) may start using the box, or may be pick another corner of the cage as a toilet. If this is the case, then move the litter box to the area your rabbit seems to prefer. Flexibility on litter box placement may be necessary both in and out of the cage.
Once your rabbit is using the litter pan in the cage, allow the rabbit out of the cage in a limited area. Provide a litter box within this area, and perhaps make it enticing by placing a a treat or favorite toy in the box. Watch your rabbit for signs he is about to urinate or defecate (they usually back up and lift their tail slightly), and try to herd him to the box immediately (if your rabbit is very calm about being picked up it should be okay to place him right in the box). If your rabbit uses the box, give the rabbit a treat (food, toy, petting, or praise) right away. If you notice your 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. Your rabbit will absolutely not be able to make a connection with physical punishment and elimitnating outside the litter box. If you catch your rabbit in the act calmly and gently take him or her to the litter box immediately. But, if your don't physically catch your rabbit urinating or defecating, it is too late for your rabbit to make the connection. Just clean up and watch your rabbit a little more closely next time (clean the spot diluted vinegar, or a commercial pet stain/odor remover). The key is to get your rabbit to the box before he goes, so a trip to the litter box every 10 minutes during playtime can be helpful.
Over time, your rabbit will probably develop a preference for using the box, and amount of freedom you give your rabbit can be increased. You may need to provide more boxes as you allow your rabbit acces to more space (rabbits may not go far in search of a box so have them handy). Again, if your rabbit repeatedly chooses one place in he room to eliminate, consider putting or moving a litter box there. Try to work with what your rabbit naturally wants to do, but if the location they "choose" is inconvenient, you can try putting a litter box there for a while and then gradually move it to a better spot. Sometimes, placing a bowl of food where you don't want them to go works too.
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. Sometimes a previously trained rabbit will get a little careless, and this usually means backtracking and restricting freedom until your rabbit is trained again.