We do all the things 2gypsies suggested. In addition, we cover the toilets with plastic wrap. Lift up toilet seat, seal bowl with plastic wrap, put down seat and lid. We have very little evaporation with this method. Never had a problem with traps (for smell or what?), though I've heard people suggest running the water. Though one can't run the water if the water is turned off.
If you use cable for TV and/or internet, check to see if they have a vacation hold. But watch out if you're on a special package price. Ask questions.