I buy hops by the pound and store in the freezer between uses.
- Remove as much air as you can. I seal in ziploc bags and squeeze the air out
- Keep them in the freezer
- Realize that hops will lose some bittering power over time. Good software can help you estimate the impact. If you them up in two to three months, the change will be minimal.
Buying in bulk means I keep hops for the better part of a year. I have used three year old hops in beer that turned out okay. The best thing to do is trust your senses. If the hops smell fine they should be good to use.
There is at least one style of beer that is made with very old, cheesy smelling hops: straight lambic. You can always try one of those.