How to stop form submit when onClick event returns false

I have the following code for my submit button

<input type="submit" name="action" value="Save Article"
onClick="this.value='Saving...'; performPrePostChecks();">
[snip]
How come that irrespective of the function performPrePostChecks
returning true, or false, that the form is still submitted?

Folks

I have the following code for my submit button

<input type="submit" name="action" value="Save Article"
onClick="this.value='Saving...'; performPrePostChecks();">

I have tested performPrePostChecks() - Its a long script, but I have proven its success by having an alert box echo the success with a simple true or
false message like so:

<script type="text/javascript">
function performPrePostChecks()
{
if(checkRequiredFormFields('form1','headline','Hea dline')==false)
{ alert("false");
return false;
}

alert("true");
return true;
}
</script>
How come that irrespective of the function performPrePostChecks returning
true, or false, that the form is still submitted?

Can someone point me in the right direction?

Thanks,
Randell D.

onClick="this.value='Saving...'; performPrePostChecks();">

To clarify Eric's correct comments, you want:
onClick="this.value='Saving...'; return performPrePostChecks();"

regards,
RR