Use Pokemon from the equal or lesser tier that you're battling within.
No two Pokemon of the same Species are allowed (No Lucario Armies, sorry).
Pokemon that aren't fully evolved are banned.
Only one Pokemon may be put to sleep at a time.
No two Pokemon may have the same item.
ONE-HIT KO CLAUSE:
No One-hit KO moves are allowed (Sheer Cold, Horn Drill, etc.).
No Double Team or Minimize.
"HAX" ITEMS ARE BANNED:
No Brightpowder, Quick Claw, Focus Band, or King's Rock.
If both players have one Pokemon left, and one uses Explosion, KO'ing both Pokemon, the user of Explosion loses.
Same as the Sleep Clause, except with Freeze.
These are some common ones, so just ask your opponent what clauses they want to use, and battle accordingly. Always organize a battle on Wi-Fi before you just run into it. Otherwise, you'll probably do something illegal. =]