Exercises to keep back straight when snowboarding?

You're gonna hate this, but it's an excellent exercise, both for keeping your back straight and your knees bent. You need to sit against a wall with your knees bent at a 90 degree angle for as long as you can, up to 3 minutes (I can last one minute). That's the stance you're supposed to take when you snowboard. Past a certain point, you'll get used to it.