iSoaker has some tips on cleaning water guns
As for which cleaners to use, I don't know much about how well different ones work, but you can check a chemical compatibility chart to make sure the components can handle the cleaner. Here's a chart for latex tubing
which should be the same as the bladders. Soap gets a B rating. Salt water gets an A. Isopropyl alcohol gets an A (seems pretty harsh to me, though). You should check these charts for every component the cleaner will come in contact with. It's not exactly clear what the o-rings are made from, but I'd assume Buna-N. I'm also not sure what types of plastic are generally used, so this is something I would figure out before cleaning.
Unfortunately, manufactured water guns probably were not designed to last as long as they have, so these problems might be unavoidable. You should consider replacement of components if they have degraded too much, e.g., tubing, O-rings, and bladders (with balloons or latex tubing).