When you get into a good position before you start your swing, it's much easier to stay in a good position all the way through. A lot of players don't understand what it means to bend at the hips. They end up slumping forward from the waist instead.Hold a club across the points of your hip bones, just under your belt buckle. Holding the club at both ends like this also pulls your shoulders back, which locks in a straighter spine angle. Pull up and back on the club, and bend forward from the hips. Now, turn back with the club across your chest, making sure the shaft points just above the ball. If it points at or below it, you're dipping instead of turning.
A strong lower body makes it a lot easier to create and maintain an athletic posture. Power squats can help you feel the big leg muscles you need in a good swing, and they're a great way to warm up before a round (or before breakfast). Balance on the balls of your feet and bend down, keeping your back straight and abs flexed. As you go down, pump your arms up, and as you stand up, pump your arms down. Three sets of 20 will warm you up nicely.