function pageLoad(sender, args) {

    var rm = Sys.WebForms.PageRequestManager.getInstance();

    rm.add_initializeRequest(initializeRequest);

    rm.add_endRequest(endRequest);

}

function initializeRequest(sender, args) {

    //Disable button to prevent double submit

    var btn = $get(args._postBackElement.id);

    if (btn) {

        btn.disabled = true;

        if (btn.className == 'button')

        btn.className = 'buttonDisabled';        
        btn.style.enabled = 'false'

    }

}

function endRequest(sender, args) {

    //Re-enable button

    var btn = $get(sender._postBackSettings.sourceElement.id);

    if (btn) {

        btn.disabled = false;

        if (btn.className == 'buttonDisabled')

        btn.className = 'button';
        btn.style.enabled = 'true'
    }

}