Make Unobtrusive Work All The Time?

I'm working on implementing an unobtrusive javascript where I use code instead of an HTML hook to call a function.

Old way: <input type="text" name="username" onBlur="javascript:function();" />

Now I have changed it to:

window.onload = function()
{
if(document.getElementById("username"))
{
var usernameInput = document.getElementById("username");
usernameInput.onblur = checkUsername('username','ajaxResponse');
}
}

which is placed in the body section of the page. Anyhow, my problem is that it works fine, but only once, right as the page loads. I have tried removing window.onload, but then it doesn't work at all. How can I get this to work every time I blur from the text field?