1. Boom. FFA always goes to heroic if not mythic and you'll need to max your pop limit in the event you get doubled. Don't threaten enemy territory until you are SURE you know who you'll be attacking.
2. Choose your enemies wisely -- NEVER attack more than one opponent at a time. Your attack should be swift and crushing -- have builders on hand to forward build & take settlements.
3. Wall back doors. Walling has got to be the most tedious aspect of the game, but walls discourage doubling. Gates should not be built until needed, as potential doublers will interpret gateless walls to mean you don't plan on mounting an attack in that direction. Be ready in any case to add gates (or just delete) and attack.
4. If you find yourself in a situation in which there are 2 separate 1v1 battles taking place, and the other 1v1 'finishes' before yours has any sign of finishing, dig in. Either you or your opponent is about to be doubled. Follow step #3 to minimize your risk of being doubled.
5. Keep a low profile -- don't allow your score to get too high too early in the game. Being 400-1000 points ahead of the rest of the group is the same as saying "triple me".
6. If you find yourself being tripled, and two of the attackers are clearly allies, dig in & threaten to tribute everything to the third attacker. Mathematically the odds are against you, and you'll need a political solution. Offer loads of cash to the third attacker in exchange for a cease-fire and his agreement to mount an offensive against the other two while you defend your area and reestablish your cash reserve.
7. Scout, scout, scout. You'll need to know who's attacking who, who's spread too thin, etc. Snatch up unclaimed settlements of any player who drops out of the game, as early as possible.
8. Gold frequently disappears in FFA, so trading and attacking trade are critical. When attacking, don't destroy enemy markets -- wait nearby and kill off all caravan units that come wandering near first.
9. NEVER whine about being doubled -- it's FFA, everyone gets doubled, and eventually you'll be doubling someone else anyway. While the above rules may help you avoid being doubled, it's still FFA. You will be doubled eventually.
10. When near, appear far. When organized, appear disorganized. When large, appear small. When small, flee. Etc. Etc. Enjoy.