I'm lazy so I buy the flax seeds already ground. Once the package is opened, they must be stored in the fridge and used fairly quickly.

It's probably better to grind your own. Whole seeds can be kept in the pantry until needed and are cheaper (I can buy whole seeds in bulk at Whole Foods, but the ground ones are pre-packaged). I've never actually tried to grind them, but I would guess that a coffee grinder would work well.