Check all the rods are equal lengths both sides to begin with.
Assuming you've centralised your servo horn you can then adjust the servo end point (EPA) on your transmitter. That is also assuming your transmitter has EPA. This enables you to adjust the throw of the servo lock to lock.
There's a bit more explained here
http://www.oople.com/forums/showthread.php?t=124945
There may also be other threads specific to the car your trying to adjust.