The idea of the posture is to find a position which will not only keep you well balanced but will be one which allows you to generate a good solid swing around your body. The Posture also determines how far you stand from the ball.
There is a brilliant Practice that you can use to obtain this Position and I will explain that to you now.
First of all get the correct grip on the club. Then stand at full height, with your back straight, legs straight, and arms comfortably straight (not locked) with the club at waist height parallel to the ground. The hands should be approximately 1-2 fists width from your waist (position is shown in photo1).
Photo 1


Now from this position you are going to let the club down slowly keeping the same distance between the end of the club and your waist and pushing backwards with your bottom. By keeping Your bottom pushed out backwards You are keeping your spine straight. When the club meets the floor (as shown in Photo 2 ).
Photo 2


the next movement is a gentle flex of the knees keeping the bottom pushed out, your body weight should now be concentrated near the balls of your feet (as shown in Photo3).
Photo 3


Now it is important that when you are letting the club down slowly that you do not stretch out to meet the golf ball. Wherever the club comes down is its natural position and the position from where the ball should be hit. So if you find that once you have performed this drill that You are not close enough to the ball or that you are too close you should shuffle so that the ball meets the middle of the clubhead. Making sure that You keep the same Posture position.