You can encrypt a querystring parameter, if security is your concern.
Or you can use other holders, such s p.cambell says above (session & cookie).
You could also store it in a database, and have the page you go to retrieve it onload.
Just depends on your application requirements.
Another thing I've done is to use <asp:panel>, basically using a single page as though it were multiple pages. In this way, I also have access to viewstate to hold my variables. (Whenever a user clicks 'next', or whatever they would click to goto the next page, I simply hide the panel they're on, and show the panel they want to go to [visible = true/false] property)