Hi,
You may consider URL rewrite maps. "A rewrite map is an arbitrary collection of name and value pairs that can be used within rewrite rules to generate a substitution URL during rewriting. Rewrite maps are particularly useful when you have a large set of rewrite rules and all of these rules use static strings (that is, when there is no pattern matching used). "
There is more information in the following article, hope helps:
Using Rewrite Maps in URL Rewrite Module
http://learn.iis.net/page.aspx/469/using-rewrite-maps-in-url-rewrite-module/