One other thing. Just like a car, you can have understeer and oversteer. Kits that have prebent wire gear can sometimes be put on backwards and give you an oversteer condition. Look down from the top. If the wheels are turned out at the front, that's oversteer. If wire gear, reverse them. At any rate, a little toe-in will help you go straight. It preloads the gear.
There's alot of good advice here! First off, I'd use a little expo, dual rates or just turn the rudder down and make sure the wheels aren't binding.