Years ago I was lucky enough to receive a grant for "Dry Erase Sleeves" from www.reallygoodstuff.com but they are an easy DIY project. You simply need clear page protectors and white card stock. You can make them fun and colorful by adding colorful dictape as a boarder (pictures to come!)
These type of whiteboard give you flexibility and the ability to customize your whiteboards whenever you need to. Need a number line to teach decimals and fractions? Just print it out! I know some teachers who set up their whiteboards in the beginning of the year with every whiteboard ready to go, number lines, fractions, graph paper... It's all there.
I prefer to hand out inserts as I go through the year. Students are able to keep it in their whiteboard during the lesson or unit. When the class no longer needs the whiteboard template, it goes into a student's math binder for review or help with homework.