Here's a tip I picked up a while back form the LHS owner...it's helped me quite a bit, especially when 'my tree' didn't come to the new field with me.

Before flying, walk to the center of the runway. Pick a landmark of some sort on each end that is in line with the center.

Now go back to your flying position. Hold up an arm, and point at the landmark on that end. Do the same for the other arm. What you're trying to get imprinted in your mind is the angle of your arm to your body. You should be able to stand at your poition eyes closed, raise an arm, and find it pointing directly at your landmark.

Once you get the right angle down you can go to any field or fly from any position, and know that you're pointing at some landmark on the runway's centerline.

If you'll then bank out of final on that point, with the airplane appearing to be heading straight for you, you'll be line up straight, on the center line, every time.

HTH