Post operation appending to Url rather than calling button click handler

Hi All
I have created an ASP.Net 4.0 website. I am using routing feature to keep urls clean.
one of the routes defined in Global.asax.
routes.MapPageRoute("signup", "signup", "~/signup.aspx");
Then I have an asp.net web form with some controls and a button. On button click , I am trying to save values to the DB on button click. But when I click the button it is not calling the handler rather appending control names and values to url. something like this:
http://www.xxxxx.com/signup?ctl00%24MainContent%24txtFName=matt&ctl00%24MainContent%24txtLName=dillon&ctl00%24MainContent%24txtEma ...