I've tried just about every relative path that I can think of, and none of them work.
Server.Transfer("/contact-us.aspx");
Server.Transfer("./contact-us.aspx");
Server.Transfer("~/contact-us.aspx");
Server.Transfer("contact-us.aspx");
I'm pretty sure this has to do with the fact that I'm inside of an ascx UserControl. I always get "The file (my file) does not exist."
EDIT:
Also, contact-us.aspx is not a physical file. I'm using Sitecore, and this is a virtual path within the application, and the application knows how to resolve the URL.