I try to use the scale balance features if possible but otherwise I make a roughly C shaped part out of 1/8 or 3/16 A/C ply and attach it to the surface like you would a control horn. The C hangs forward ( actually under on ailerons) over the hinge line with the opening parallel to the centerline or chord line. I make the forward part wide enough to enclose lead 'tape weights' and try to make it so it stays within the wing as as much as possible. It is smoothed of course. I try to add enough weight so the surface is about 75-80% balanced at the hinge line. I use epoxy to stick the weights. Sometimes you simply can't get that much but get it as close as possible. I also use tape to seal the hinge line on all control surfaces except flaps.

I got this a long time ago somewhere so it is not my invention. Just using someone else's good idea.

I've had and built some really fast planes over the years and flutter has not been a problem. My Z Corsair was knocking on the door of aileron flutter untill I added this feature. Servo problems went away overnight.