As mentioned above, the less "air" the better. When I receive my online orders, I break them down into smaller bags. And, for the smaller rodents, I place a few of the smaller bags into 1 larger bag. I do this so that when I take a rodent to feed I don't have to introduce air into the whole bunch.
Likewise... some people repack and vacuum seal. I bought a vacuum sealer for this purpose, but then never ended up doing it. I guess I got lazy and figured too much work when my former method was working fine.
But... however you do it, less air and preventing them from thawing is key. I buy a 6 month supply... but, as long as they stay packed airtight, I will keep them for up to a year. I've never gone over a year. Any stock left past a year from when I order them gets dumped.