<select name="array[]" id="array" multiple>
<option value="Val1" <?php if( @in_array( 'Val1', @$_POST['array'] ) ) { echo 'selected="selected"'; } ?>>Val1</option>
<option value="Val2" <?php if( @in_array( 'Val2', @$_POST['array'] ) ) { echo 'selected="selected"'; } ?>>Val2</option>
</select>
<script type="text/javascript">
var arr = new LiveValidation('array', { validMessage: ' ', wait: 500});
arr.add( Validate.Presence, { failureMessage: ' ' } );
arr.add( Validate.Exclusion, { within: [ "" ], failureMessage: ' ' } );
</script>

<option value="Val1" <?php if( @in_array( 'Val1', @$_POST['array'] ) ) { echo 'selected="selected"'; } ?>>Val1</option>