Cleaning
Rinse with water first to get rid of dust and grit. Then wipe with a microfiber cloth. If they're really dirty, use a tiny bit of mild dish soap, rinse it off completely, and dry with the microfiber cloth.
Don't use paper towels or your shirt - they'll scratch the lenses.
Storage
Use a case. Simple as that. Keeps them from getting scratched or sat on. Don't leave them in a hot car or they might warp.
When you take them off, lenses up if you're not putting them in a case.
After Salt or Chlorine
If you've been in the ocean or pool, rinse them with fresh water. Salt and chlorine can mess with the coatings over time.
If They Get Loose
Check the screws at the hinges. Tighten them with a small screwdriver if needed. Most sunglasses come with a little tool for this.
Fogging Up
Keep the lenses clean - oils attract moisture. You can also try anti-fog spray if it's a regular problem.
That's pretty much it. Take care of them and they'll last.