Generally with a login page the first thing you want to do is trace the header out to find the actual login form fields, i.e the post form variables. Then you want see if you can send data to the server using GET rather than POST. So for example if a form as 4 variables, $username, $password and $x then you set the request to:
to see if you get the wrong user/password response from the server. If so then it is much easier to find an injection vector using GET than it is using the POST method.
If that is allowed, then you could start trying to trigger database responses like
/Login.aspx?CurrentPage=Login&username=name'[enter your SQL statement here]&password=pass&x=1
If the POST method is the only method allowed then you will not be able to use the 'URL' to submit the data, but rather following on from what bstpierre stated, you will need to make up a form submitter using one of the browser addons like Live HTTP Headers for Firefox or rebuild the form on your own server to submit to the site you are testing.