To try and answer the original question, 1) If you know that the plane is prone to flutter at high speeds, you may be just beating yourself over the head trying to fly it faster.

2) Three books I've just bought after returning to the library the 8 books I borrowed are: "Simplified AircrAft design for homebuilders" by Dan Raymer, "Model aircraft aerodynamics" by Martin Simons and "Basics of R/C Model Aircraft Design" By Andy Lennon.
All three discuss flutter, though not in huge detail. Interestingly they all relate mass balancing to flutter.

3) Better yet, get a beginning electronics book and read the part about resonant circuits. Circuits of resistance combined with capacitance or resistance combined with inductance. The math is extremely simple. Then go out and look at the spring, shock absorber system attached to each wheel of your car. That's a mechanical version of a resonant circuit.

Now the deal is, flutter is caused by an aircraft surface being inadvertently designed as a resonant system. Its designed in such a way that under certain circumstances, (in Spoiler's case excess speed) energy gets added to the system at the resonant frequency. And just like when a fool keeps adding just a little push to a child's swing at the swing's resonant frequency, the child goes higher adn higher until the swing wraps around the bar or the kid shoots out of the arc or whatever.

In that same way, the control surface picks up more and more energy from the airflow and buzzes louder adn louder until the part/plane fails.

The ONLY 2 cures for this are to change the resonant frequency (strictly speaking, you broaden the frequency range where the system will NOT resonate) or remove the energy input or alter the input frequency.

Mass Balancing is just that. You balance the mass of the surface around the hinge. While one way to address flutter, it needs to be done based on proper consideration of the principals behind the problem.

Making the parts stiffer or the linkages tighter also, while being valid ways to address the problem, if not done in combination with proper understanding of the underlying theory, you may just be altering the resonant frequency. The plane might just self destruct at a higher/lower speed.

Anyone who's seen film of the Tacoma Narrows bridge knows strength or stiffness ALONE is meaningless. It was a huge, modern suspension bridge that rocked itself to pieces because the WIND reached the right speed. It had nothing to do with gusting. The turbulence around parts of the structure resonated at a certain wind speed. It rocked the bridge back and forth until it ripped itself appart. Until you see a good demonstration of resonance, you can't believe how little power it takes to destroy something if that little bit of energy is just added at the resonant frequency.

Sorry, I can't be of more practical help. Like I said, the electrical math is easy so its easy to learn an understanding of what's going on. Understanding a spring/shockabsorber combination gives better insight into mechanical resonant systems. You're looking for the opposite of a band pass filter in electronics. A combination of weight and or stiffness (shock absorber type of stiffness) should do it, but how to find the correct combination to dampen a frequency range, I couldn't tell you.