Bathroom exhaust fan turn on with light or independent switch
Install a humidity sensor fan control, like this one from Leviton. It will automatically turn on the fan, when it senses excess humidity in the room.

humidity sensor fan control http://www.homedepot.com/catalog/productImages/300/43/43ea781d-e83f-41d3-98c2-90fec341f3c6_300.jpg

You should be able to find them from other manufacturers as well, this was simply the first example I found when searching.