Well.... the way most systems do it is to do a lookahead... check to see if you can win in n moves or your opponent can, and take the best path. This is pretty easy in tic-tac-toe do to the simplicity of the game. Of course, as long as you play in the corner in the first move (if you are second) or the center (if you are first) then it is very unlikely you will be trapped.
                - Ant