Incline Flies!
Any and all incline movements for that matter.
If you happen to look at most peoples chest, their weakest point seems to be the upper chest. You are likely afflicted with the same problem. Yes--that's why you can flat-press more than you can incline, and decline more that you can flat.
At least every other chest workout, start with an incline movement, and ALWAYS do at least 2 upper-chest movements in any given chest workout. (incline presses, incline dumbell presses, incline flies, and neck presses are all good options). By starting with upper chest, and thereafter making a deliberate effort to concentrate on the weakest portion of the chest, it should eventually bring everything up to speed.
I like to call it "Trickle-Down-Pecanomics"
--moe