If you don't mind the price tag, "The Ultimate Tone", a book by Kevin O'Connor has a whole section dedicated just to switching methods.
RG Keen(forum member RG) also has a neat solution for two channel switching, mostly used in effects pedals but usable in amps as well: The Millenium Bypass
Other than that, you can dive into existing schematics and see how they do it. There are tens of types of channel switching solutions, maybe start off a current production amp like the HotRod Deluxe or similar.
Hope this helps.