I have found that there are a couple of things that will slow or stop wingrock altogether.
When performing an elevator too much throttle or not enough elevator throw will cause wingrock,,,, see if backing off on the throttle helps. Also a spoileron mix will help too. That means a mix that moves the ailerons up when up elevator is applied, start with about 15 degrees of up ailerons when full 3D up elevator (50-70 degrees) is applied. With the mix the ailerons will move up OR DOWN the same direction as the elevator throw. Doing this kills some of the wings lift which is the goal of an elevator, The wings lose lift but trememndous elevator throw keeps the aoa up enough to keep the nose from dropping. Reducing wing lift stops the wings from rocking in an elevator in my opinion. IMO, Rocking is caused as one wing loses lift and then the other back and forth. Stop them from fighting each other by bringing the ailerons up, but not so much as to cause total lift loss and stall. Its a balancing act. Adjusting CG and the other thing mentioned in this thread help too, but these are some other things I have found.

I have a question..
What is it about my foamie biplane that it drops like a STONE and im talking rapid vertical descent in an elevator but absolutely no instability WHATSOEVER. Go to high rates, chop the throttle apply full up elevator 70degrees and that succer comes straight down, if theres no wind its PERFECTLY flat and a nice smooth but rapid vertical descent. If the breeze picks up just a little, maybe 2-5mph I can land an elevator the descent is so slow and predictable.. What is the difference between my foamy bipe and my glow planes??My U CAN Do does really nice elevators with a spoileron mix, but my foamies are just sick!!!!

Harriers or rolling harriers I use a flaperon mix, really stabilizes the plane,,,, no rock.. and rolling harriers are about twice as easy to control with flaperons. Flaperons are exact oppsite of spoilerons. Elevator goes up Ailerons go down and vice versa.

Mike