Have you ever had any injuries? Or do you perform some sort of work every day that would lead to this imbalance ie. are you a carpenter?
If it's an injury thing then I'd reccomend ART (active release therapy). I've just gotten my 3rd treatment and would reccomend it to anyone who's even remotely interested, it works well and you notice the results quickly.
Other then that, maybe something like mullet said. Except instead of working "just" the smaller trap, try working it first and then whatever you fail at, say it's 8 reps, you only do that number of reps for the stronger side. Perhaps even do 1-2 less for the stronger side. I've done this for my legs before when I've had issues with my knees or a shoulder is bothering me. It's good to bring up a lagging bodypart.