The easiest solution is just to put a small, tiny little disable function right inside your form. We use this for the e-commerce sites we build and it works great -- no multiple clicks yet! 
<form .... blah ... OnSubmit="this.my_form_button.disabled=true;">
<input blah>
<input blah>
<input type="submit" name="my_form_button" value="Click Here">
</form>

[download]
As soon as they click the button, it is automatically disabled, and they cannot click it again (even if they are superman).

Michael Jensen
michael at inshift.com
http://www.inshift.com

In reply to just disable the button! by inblosam
in thread Avoiding a second click by MMilani

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