Stale weed can give you a mad headache, but it usually doesn't make you higher or give you a different high - it usually, if anything, gives you barely a high at all.

Never keep your weed in a plastic baggy dude. It'll dry out in a day and the plastic fucks with the crystals. Not only that but you can always smell it on you.

Keep it in a glass mason jar or metal airtight tin, like the others said!