1. NEVER, but NEVER dig down.
2. Don't pretend you are a hero when mobs are nearby.
3. When mining, ALWAYS look before advancing(you may fall in a lava pool).
4. If you see a Creeper, run, unless you have good armour or you can fight very good. Not everyone can handle creepers.
5. Never dig above you.
6. When digging or mining, wait 2 seconds before you eventually go further.
7. Never understimate gravel or sand.
8. Don't go into The End unless you are prepared to die.
9. If it's your first time in the nether, don't bother the Zombie Pigmen.
10. Always bring a bow and a stack of arrows in nether. Ghasts are annoying.
11. Ignore Endermen, unless they attack you.
12. NEVER try to hit an enderman with an arrow.
13. If you are on a naturally created Obsidian surface, rule 1 applies.
14. Try to swim in lava if you have Fire Resistance enabled.
15. Have fun.

I'll probably update with more when I get more ideas :)