Well, I haven't actually done this part...yet. But as far as adhesive goes, I've heard hot melt caulk, ABS Cement or 2-part epoxies all work well.
Like Brak's said though, when the time comes, I plan on screwing my visor in...it's really the only way to go if you ever need to easily replace your visor.
I have read that you can run the screw all the way through the front of the helmet, trim it off, sand it down and use bondo or something to cover the screw post and hole. But I don't think I'll go this route.
What I might try is to affix a raised piece of plastic, fiberglass, etc. to the areas inside the helmet where I plan to attach the visor screws. This will create nice thick solid points to attach to.