How To Add Multiple <option> Values From A <select> Into A Text Box

What I'm trying to do is get multiple options from a <select> list, and display the values in a text box. So here's my select:

<select name="js_strPreDefCatList" size="6" multiple="MULTIPLE" class="formtext">
<option value="All content">All content</option>
<option value="Region: Africa">Region: Africa</option>
<option value="Region: All Americas">Region: All Americas</option>
<option value="Region: All Europe">Region: All Europe</option>
etc.....
</select>

'// Followed by my button

<input name="Add1" type="button" class="formtext" value="Add" onClick="addPhrase()">

'// Then the TEXTAREA

<textarea name="js_strMemberMail" cols="40" rows="1" wrap="VIRTUAL" class="formtext"></textarea>

The function "addPhrase" is where i know i'm having the difficulty - so for now I've forced it to show one value only:

'// The form name is AMANDA

function addPhrase() {
document.AMANDA.js_strMemberMail.value = document.AMANDA.js_strPreDefCatList.value + ", " ;}

What I think I want is to be able to build up a string of values and write them out to the textarea (hence the ", "). With the option that more can be added, without deleting what's currently there.