dynamic page to friendly url

Hi

I am trying to convert the following pages to something more seo friendly.

mypage.aspx?id=8

to

mypage/8/

however the rule I am applying is not being actioned.

i have entered this into the web.config file

<rules>
<rule name="Rewrite to article.aspx">
<match url="^mypage/([0-9]+)/" />
<action type="Rewrite" url="mypage.aspx?id={R:1}" />

</rule>

thanks

andy