First make sure you have a Pomeg Berry and a Pokemon who has not had their HP lowered as much as possible already through Pomeg Berries. (If the latter is the case, the use of HP Ups on the Pokemon will be enough.) Get the Pokemon to a very low level of HP (1 or 2 HP). Once done, in the field, use the Pomeg Berry on the Pokemon. This will in turn cause the data in their HP to cycle back to the top of the counter, giving the affected Pokemon 65,535 HP--almost 90 times the possible maximum in the game.