I've never seen one in normal residential plumbing but in commercial plumbing I've see lots of trap primers. This is a funky device which attaches to the cold water line at (usually at it's highest point and mounted vertically). A line runs from the primer to the trap. Every time there is a quick change in water pressure it gives a little squirt in the trap to keep it full of water. Something like this could be your solution unless you want to pour a cup of water in your shower drain every day or two to replace any evaporated water.