Move First
Moving first will double your chances of winning. Over the course of the game, the player who goes first will claim five of the nine boxes on the grid, leaving only four for the opponent. This gives you many more opportunities to win.
Use Safe Moves
Your first set of moves should be the corners or the center spaces. These are known as "safe moves" as these positions allow you the greatest chances of winning while still retaining control of the other available boxes. Never start on the edges-- these spots are the least advantageous and will allow your opponent a greater chance to win.
Avoid Random Plays
While Tic Tac Toe is a relatively straightforward game, pay attention so that you can successfully block your opponent. The less attentive and more random your moves are, the greater the chance your opponent has of taking advantageous boxes, thereby winning the game.