Disable button after click and display text [Processing...]

Good, my solution is by using Client Javascript

.aspx page

<asp:Button ID="btnSave" runat="server" Text="Save" CssClass="btn DisableWhenClick"
                    CommandName="Save" OnClick="btnAction_Click" />

script //by using jquery

function DocumentReady() {
    //Button disable when Click
    $(".DisableWhenClick").click(function() {
        $(this).attr("disabled", true);
        $(this).val("Processing...");
    });

OK, then any button you need to disable when submit, you just need to add class "DisableWhenClick"

Enjoy!