Dumbell Presses instead of Barbell presses. If you're already doing both switch the order you're doing them in. Many guys who do both usually do the Dumbells as a suppliment to the barbell presses and go lighter on the dumbells. If that's what you're doing, do the opposite, make your dumbell lifts the heavy ones and go easy on the barbell work for a while.
You'll probably find that this helps even you out and the change could help you add some extra muscle as well.
While perfect symetry might not be necessary, it seems this lop sidedness is enough of a concern for you to post about it so you should probably take some action to correct it.