In Black 2 and White 2 the odds of passing down the Nature of the parent holding an Everstone is now 100%. If both parents are holding an Everstone, the odds are 50/50 between each of the parents' Natures.
If you have a parent with the nature that you want, give it an Everstone to hold and it will pass down its nature to the offspring.
If you don't have a Pokemon with the correct nature to give an Everstone to, then the next best way is to soft reset for your desired nature, or you could look for a Pokemon with that nature and breed it.
Just remember that the female parent determines species, so the male parent can be any Pokemon in the same egg group as the as the species that you want. It might be harder to get a female Pokemon with the nature that you want, since you will need one that's the species you want to breed, while you will have a larger selection of male Pokemon to find the right nature from since it can be any species (in the same egg group).