The key to winning an FFA is figuring out a way to fight the LEAST. 9 out of 10 times the player that wins is the one who is left alone the longest. So consider these rules:
1) Don't insult anyone in chat. Don't whine or complain. In fact, not talking at all works well (this is of course if you are really concerned with winning, half the fun of an FFA is yelling at eachother IMO)
2) Manipulate others into fighting eachother. Nukes are great for this but useless otherwise. The truly gosu among us can take two small armies, attack to players with inferior forces, then retreat to the middle of the map somewhere, drawing the two bigger armies into conflict with eachother.
3) Mass, don't harrass! Harassment pisses people off. Pissing people off makes them want to attack you. This is BAD.
Of course, there is one other plan, sure to garner laughs, cries, curses, and other exclamations, but almost never wins the game. Play zerg, turtle behind tons of sunkens and get 200 supply of 3/3 cracklings. Then just attack-move in every conceivable direction and bask as streams of lings light up the mini-map in your color. Make sure your basking is good because about .5 seconds later every other player will gang up on you and kill you.