The middle is prime real estate: one mark can work in several directions there. A long, obvious line is easy to squash, so try crossing two smaller threats instead. And yes, check the computer after every move. Its open three may look harmless right up until it isn't.
- Keep your marks close. Lone pieces rarely save the day.
- See an open four? Block it. Right now.
- Make two threats cross and force the computer to choose.
- Leave breathing room at both ends of a useful line.
Why 10x10 gets interesting
Regular 3x3 Tic Tac Toe is basically solved before it starts. On a 10x10 board, plans can grow, collide, and fall apart in a couple of moves. The rules are still dead simple; the board just gives them room to cause trouble.