First, find out if there are any open spaces by looking for links saying 'open' on the main site. If there aren't any, you'll just have to wait for some. Here are the guidelines for your site, if you're not a friend of mine (I'd probably have linked you already):
Must be related to Harry Potter.
No Redirection URLs.
No bad graphics, language, etc.
Must have a good layout.
Must have at least ten subpages.
Must have link to my site first. If I go there and it isn't on there, I'm not linking you. If I go there, and it is on there, but I decide not to use your site, you can take it off.
Just email me at webmaster@harrypotteravenue.zzn.com asking for a link exchange, and I'll reply as soon as possible with my answer. If I do not want to use your site, I will not reply.