You can move one seat to the left every two or three turns. Only keep life totals, leaving the rest to the next guy. It keeps things honest, as the killer combo and/or Plague Rat deck turns on its builder, instead of everyone else. (Being attacked by 11 11/11 Plague Rats is NOT fun!) Lots of fun with the assorted "floating" cards. This also allows insight into how your friends think about deck building.
Another thing to try -- build one large (250+) 5 color deck, and have everyone use that deck only. Take turns drawing cards for your initial hands, then draw away. For greater amusement, make the deck a highlander variant -- but throw in a Coalition Victory, just in case...

Deck construction is either one person's cards, or just combine your decks (if you trust each other, if only somewhat, just list what cards you are contributing to the pile, for later recovery).
Maybe just play regular highlander, with chaos rules. This will probably result in teaming up, but games will last longer due to lack of reliability.
Though better with more people, the hunt can work, and forces people to pick one opponent, and one opponent only.
Change victory conditions, like the first person to reach 50 life loses... games (and decks) change immensely when trying to give opponents life (a GREAT use for the white wind!).
Build a 40 card deck of global enchantments and artifacts, drawing one at the start of each third time around the table. During the time that card is in play, everyone is affected, and the card in question CANNOT be removed, tapped, or otherwise ignored... A great use for those single copies of cool cards (Howling Mine, Mana Flare, Naked Singularity, etc.) you have lying around. This also forces creative thinking, as players try to work around the cards in play. Ensure painful as well as beneficial cards.
Bubble Magic might prove interesting. Basically, everyone gets a predetermined number of turns to play without being able to be attacked or have anything targeted by anyone else. The drawback is that comboes rule in this format. This is also how my friend killed 8 of us using Channel, Hurricane, and a COP Green.
These variants can also work with more than three players, and in fact are more fun that way.
Enjoy!