The Sig Four-Stars are somewhat prone to flutter, largely, I think, because their wings are not very torsionally stiff. Keeping control surfaces as lightweight as possible helps - note how many WW2 fighters used fabric covered control surfaces.

I have noticed that kit models that have rather dense balsa in their control surfaces fall victim to flutter far more often than those with 4-6 pound per cubic foot stock. The best fix by far is mass balancing the control surface, but putting excess balance weight at the tip of a control surface can make things worse, as Mr. Matt says. If you use a single balance weight at the tip, it should counterbalance only about a third of the unbalanced moment of the control surface; otherwise you run the risk of exciting higher harmonics that can rear their ugly head and tear your model to pieces in a split second, even after many hours of flutter-free flying. A single balance weight, located at 60-65%% of the length of the control surface from its inner end is better, although a bit inconvenient, and it should counterbalance about 80% of the unbalanced moment.