Like Warped said, give it a blast at 30psi or so to seal it. Note that the beer won't be carbed when you swap it in, so you'll still need to let it sit (or force carb) when tapping it. Or, if it's going to be on standby for a bit, you could toss in some priming sugar and let it naturally carb while in the keg (should still give it a blast of CO2 to seal it).
Atomic Dog Brewery
On Deck: Centennial IPA, Dry Irish Stout
P: Cascade Pale Ale
T: Carbed Crystal Light, Root Beer, Vanilla Cream Soda, Tripel IPA, Apfelwein
e^(pi*i) + 1 = 0