I now present, for your enjoyment, the Ultimate Tic Tac Toe Strategy Guide:

Tic Tac Toe Theory
First of all, given two players who are fully aware of the repercussions of every move, this game will ALWAYS end in a tie. However, most human players are not that calculating. Presented with a situation where there are no blocks to be made, and no way to move toward completion of a line, you'll often find people who just plop an X or O in any square that suits their fancy. After reading this Ultimate Strategy Guide, you'll be able to beat them silly and assert eternal control over their immortal souls.

Basic theory goes like this: if you have two symbols in a row (for instance, opposite corners, a side and the center, etc), you'll only avoid having it blocked by deliberate stupidity on the part of your opponent. Thus, to win you must set up a situation in which TWO such possibilities simultaneously present themselves. Your opponent can block one, but then you can complete a row, winning the game.

A note about terminology: 'sides' refer to the squares with three bordering lines. 'Corners' are the squares with two bordering lines. The 'center' is the square with four bordering lines.

Tic Tac Toe: First Move
Assume your opponent is smart enough to block any two-symbol line set-ups, but not analyzing enough to premeditate the possible game outcome. If you are playing as X, your best case scenario is a win, and worst case scenario is a tie. As O, you can always force a tie, but winning is going to take some deliberate stupidity on your opponent's part.

If X's first move is... than the best case scenario is...
• Center: Tie if O takes a corner (50%), win if O takes a side (50%).
• Side: Tie if O takes a corner or center (63%), win if O takes a side (37%).
• Corner: Tie if O takes the center (13%), win otherwise (87%).
This demonstrates the possible first moves, and the appropriate white counter.

Tic Tac Toe: The Aftermath
From here, it should be fairly obvious where to go. As X, you're trying to set up a dual-row scenario. Play some 'toe against yourself if you can't figure out how to force the win from a scenario listed as a win. As O, you're trying to prevent a dual-row scenario by forcing X to block you. Again, play some 'toe against yourself if you can't figure out how to force the tie from a scenario listed as a tie. Good luck!