Oxygen is the problem for alot of glues and finishes. You can get a product called "Bloxegen" that is sprayed into the can/bottle before sealing. This will displace the oxygen and HELP keep you product fresher,longer.
An oldtimer at the Hardware store I grew up working in told me to exhale your breath into a can before sealing for a longer period of time. The c02 in our breath will also displace the oxygen.
And remember Erics trick to 'squeeze" out the extra air if you can. Storing upside down as Bue suggests, works as well. Be sure the can stays sealed though, I ended up with some sealer all over my bench before. Still not sure how that happened.
CTT