Keep a copy of your prescription so you can buy glasses online for cheap.
Glasses fog up when you go from cold to warm. Clean the lens with soap water and wipe them off to keep them from fogging up. Or use good ol fashioned spit.
Check the screws and whatnot on the hinges periodically. Sometimes they go loose. Depends on the glasses.
They help keep rain and midges away from your eyes, so that's nice.
The nosepads get dirty too, so wipe em down every now and then.
If you play sports and you don't care for contacts, or if you swim, think about getting prescription googles. 100% worth it.
Take them off on the rollercoasters. Put them in your pocket or something.
Buy a spare pair in case of emergencies. This is equivalent to not having a plunger when you need one. You're gonna regret it!