I know JavaScript, but not very well, so do something like this:
<form method="post" action="submit.php" name="form1">
<select onchange="documentforms[0].form1.submit()">
<option value="something">something</option>
</select>
</form>
Now, I'm not 100% about the JavaScript but I'm sure it's something along those lines...maybe someone else could help me out.