Sounds easy right? Well no....

 A server side button's OnClientClick is no good because the form's validation runs after the code assigned to this property is run. This means that you don't know whether the form is actually submitting until its too late so there's no way to know whether to disable the button.

 The form's onSubmit is also no good because there's more asp.net framework code running there.

 Any ideas?

Cheers, WT.

Oh cool, assigning code to the form's 'onsubmit' attribute has asp.net add the code after all of its processing. There it only runs if the validation has succeeded.

I think this might give you your answer:

http://www.davidbreyer.com/2005/06/08/build-a-better-submit-button-in-aspnet/