I use scrapbooking paper for my backgrounds. Lots of options for looks and cheap! Can't beat that. Actual backgrounds have gotten ridiculously pricey lately.
For tanks, just tape it to the outside. For other cages, glue it to the back then add clear vinyl or another clear plastic over it.
**Adriana - White 'N Nerdy!**
1.0 BP 'SunSpot', 0.1 Corn 'Freya', 1.0 IJ BTS 'Topaz', 1.0 ND bunny 'Licorice'
Search Amazon.com for "art poster print" followed by a word describing what you want to see. For example if I wanted a forest, search "art poster print forest". They have a ton of cheap posters that you can use that are way nicer than your average pet store background. This is what I did for my glass tanks, and it is working out well for me.