If you're looking for a freebee, start by adjusting the rear spring preload to achieve 3.5 inches of rear suspension sag when you sit on the seat.
Also, you should lube the rear suspension bearing/collars/seals from time to time.
Beyond that, go to the Suspension forum and do some reading there.
More than a couple WR-250R and WR-250X riders have sent their fork and shock out to a suspension shop for work done to the hydraulic valving (called a "revalve") and different springs if needed.
I sent the rear shock of my WR-250X out to a guy on here called "Mr, Go Race".
His shop is called Go Race suspension.