With Inov8's H20 bladder you never need to clean it, the website says it's bacteria free. I'm not sure that I believe them though, so I try to clean it by putting my false teeth water in it and swilling it around as others have mentioned. I've not put it in the freezer yet, but I can see the merits. If I fill it with water first it'll last a while longer on the run as well.
Seriously though, I can't for the life of me get this bloody thing dry. Anyone found a good way to do so?