You can't, variables in XSLT are not like variables in other languages, once they're set they're set and you can't increment them in the way you would do in PHP for instance. However, you can set a variable to something that will change, like position() - as an example:
Yeah, that is how I sued to have it. Something like position() % 2. But, not all of the XML elements are in sequential order, so I don't get even coloring. Hrm, thanks for the relply though. This is kinda what I thought.