1. Open Discussion Board -> NewForm.aspx using SharePoint Designer
2. Paste following javascript code to bottom of the “MainContentPlaceHolder”
<script language=”javascript” type=”text/javascript”>
var d = document.getElementsByTagName(“textarea”);
for( var i=0; i < d.length; ++i){
if( d[i].id.indexOf(‘TextField’) != 0 ){
d[i].value = ” “;
}
}
</script>
Dilhan's Blog 
If anyone actually tries this, you have to replace the double and single quotes when you paste this into SP Designer. In their current form they are a unicode character and that won’t work in javascript.