When I'm putting my gear away, I use a condiment squeeze bootle to flush water through the p-valve. Some people add vinegar, a little bleach, or a disenfectant solution.
I use mouthwash, because of the alcohol and the fresh sent.
I saw some extra large bottles of the mouthwash at the dollar store one day, and so I bought 4 bottles. As the cashier was ringing me up, she took a sideways glance at me and muttered, "Yeah, cheap way to buy alcohol, huh?" I guess some drunks drink mouthwash. I tried to think of a way to explain about the pee valve and everything, but just stood there looking stupid.