Weight distribution is key as well. Figure out how you're most likely to tip. In this year's competition for example, many robots would be prone to tipping backwards driving up the 6" steps. As was the case with our robot; in fact, in Pittsburgh, it fell backwards on its own without anyone else pushing it. How did we fix it? Moved the battery from the back of the bot, to the front
But certainly; general rule of thumb is keep the centre of gravity as low as possible. And if you have to raise that centre of gravity to complete a task (i.e. capping a goal) try to use as lightweight materials as possible... and above all, have a gentle hand on the joystick... jerky movements will just make the robot more prone to popping wheelies.
And finally... I think above all, the best protection against falling backwards is having an excellent driver. To know when you're crossing the line with the robot and how to quickly recover if the robot starts to tip.
Kudos to our driver who managed to keep our robot vertical for this round:
http://vgamp.com/storage/783_horizontal_robot.rm