Try concentrating on keeping your shoulders back. You can do this any time too, not just when you're riding, this will help improve your posture, which in turn will help you to sit straighter when you're posting to the trot.
Also, keep your breathing regular and even. A lot of people I've seen tend to almost hold their breath when they start riding (not sure why, I used to do the same thing!)
Make sure that it's not your saddle that's forcing you to tip forward, if the saddle is lifting at the back, or not providing your leg with enough support, it's very easy to get into the habit of tipping forward.