I had the same question, when it came to my Aeroworks Edge 540 (90 size) I'm building. At first I did something really stupid, I figured if I reverse the positive and negative wires from the servo, the motor would move in the opposite direction. WRONG, I saw smoke and my brand new servo was fried. Then had each control arm facing opposite directions (one facing up and the other facing down) on my two servos. Went to the hobby shop and asked how some of the guys reverse there servos. He said !QUOT!you need a Y adapter reverser!QUOT!. The one I got is made by Maxx Products, mine was $17.50. It has a small circuit board with a fine adjustment for one side to synchronize both sides.

I hope this will help you. What ever you do don't reverse any wires.