If you're not in a secure area, your rabbit must be accustomed to a harness before you start to jump. You can buy a rabbit harness (or even a cat harness, these fit rabbits too) from nearly any pet shop. There are lots of different types of harnesses, the most common is the 'H' harness, which looks like the letter 'H' turned on its side, there's a strap round the neck and one round the stomach, with a strip in between which joins them together. You can also buy 'Figure of 8' harnesses which have a similar design, a strap round the neck and stomach. 'Shoulder' harnesses are very popular for jumping rabbits; one strap goes round the chest rather than neck, and the other around the stomach. There is also a 'Chest' harness, which from underneath looks like a 'H' harness on upside down. 'Shoulder' and 'Chest' harnesses are not found in pet shops though; they must be made or bought from abroad. A popular shop to buy shoulder harnesses from is www.rabb-it.se.They are very well designed and the harness can be made to fit your rabbit.
The younger the rabbit, usually the easier it is to get them used to a harness. It should not smell of another rabbit, and will have to be adjusted to fit the rabbit. It's important that the harness isn't too tight or too loose, you should be able to fit about two fingers under the straps. If the harness is too tight the rabbit will be uncomfortable and likely to get stressed, if it's too loose then the rabbit can easily chew the harness or even slip out of it. If the rabbit tries to nibble the harness, try distracting them with some food. Some rabbits aren't bothered by a harness at all; others take a while to get used to it. You could also try just laying the harness on the rabbits back so they get used to the feeling or putting the harness on for a very short time if the rabbit is uneasy. If your rabbit really isn't taking to a harness, try finding a very light-weight 'rope' one. You must be careful with these though as the thinner the harness, the more it can hurt the rabbit if pulled at. If the rabbit does not take to the harness at all, you can still jump at home in a secure area, or at a fenced competition.
Harnesses must be worn at outdoor competitions. If a rabbit tries to run away, they can easily be stopped if wearing a harness and lead. We don't want rabbits getting lost, or mating/fighting with others. The rabbit must have a good time in the harness; otherwise it will see the harness as a negative thing. You should NEVER lift a rabbit by the harness, and avoid pulling on it. If the rabbit runs out of your reach, slowly tense up on the lead but do not harshly pull on it.