This is the first entry in this category: AJAX (style) guide. Here I will try to sum up some of the main concepts that AJAX provides nowadays, as I see them. Most of the things might be really tiny problems and their solution - very easy for copy and paste. So enjoy.
Nowadays it is already a standard to change the submit button’s text right away to something like “Sending…” and disable it. You can even add a little hour-clock icon or something alike beside it. This way the user sees that the form had been submitted, no matter how long it takes, the user won’t press submit again. Because the button is disabled, he can’t. But most important he gets a visaul response that the form is on it’s way, so now he implicitly knows that he only has to wait.
Here is the example, just try it and click the button: