Hops do fade over time and you can't prevent it. One thing you can try is taking some whole leaf hops and a coffee press (French press) and doing your own wet hopping from the keg. Put some hops in the press, fill it with beer from the keg, let it steep for a couple minutes then press the plunger down and pour the beer into a mug. I know some people who have tried it but I haven't yet. I remember a bar in the news that does it too.
I drink (homebrew), therefore I am (gassy)