Remove a servo from the tail (if so equipped) and add a lead shoe to the servo case and reinstall.
Bolt lead (stick-on or wheel weights) under the horizontal stab. Best method is to open an access point in the fuselage.
Add a lead shim between the tailwheel bracket and the fuselage.
Drill 1/8" holes and epoxy nails in the rudder.
Move a battery back behind everything.
Use shorter engine stand-offs.