Instructions
Place your first O in the center of the board. This will give you the most opportunities for making rows of three: two diagonals, one vertical and one horizontal going through the center. If your opponent puts his first X in the center of the board, place your first O in a corner. Avoid the edges, which give you only two opportunities for making a row.
Block your opponent from making three in a row. Regardless of where she played her first move, a good player will use her subsequent moves to set up a win. Use your move to block the third space of the row. At the very least, this will set up a draw, where neither player is able to win.
Wait for your opponent to make a mistake. If your opponent does not place his X to set up three in a row, use your O to set up three in a row for yourself. Always go for the corners before the edges, since they will leave you another opportunity to win even if your first try gets blocked.