Clear textarea on focus

I've used the fckeditor on a textarea. i've set standard text in this area and i want the text to disappear when i click in the field. I've tried this code but the text stays ...

<textarea onFocus="this.value=''; this.onfocus=null;" name="content" cols="50" rows="10" class="box" id="content" border="0">voer hier je bericht in !</textarea>

any idea how i can clear the box ?

thx