I am doing validation for username and email from JavaScript with the server similar to the way twitter is doing. I have to make sure that I do not bombard my server for each keystroke and mouse click (in case user decides to copy+paste the email address using the mouse). This is how I did. It works so far but as always it can enhanced. I can share the full source but it is the skeleton to give u the idea. In essence I am waiting for 1300 milliseconds before hitting the server again.

$(emailTextBoxId).keyup(function (event) {

    //executes a function, once, after waiting a specified number of milliseconds
    setTimeout("CheckEmailAddressAvailability()", 1300);
    //stopping bubble
    return false;

});

$(emailTextBoxId).keydown(function (event) {

    safeToPerformEmailCheck = true;
    document.getElementById("ValidEmailAdd").style.display = "none";
    document.getElementById("ExistingEmailAcc").style.display = "none";
    //document.getElementById("InvalidEmailAdd").style.display = "none";

});

$(emailTextBoxId).mouseup(function (event) {

    safeToPerformEmailCheck = true;
    CheckEmailAddressAvailability();

});

Edit: I just saw the examples posted by fellow readers at following links are much better than mine. I am glad I really learned some good stuff today and I will edit my code today as first thing once I go home this eve. Really neat.

http://jsbin.com/ejimok/3/edit

http://jsbin.com/ejimok/5/edit