Add a rubber washer:
http://www.thesamba.com/vw/forum/viewtopic.php?t=161648

Paul Clarke wrote:
> Hi
> We have a mirror problem which is driving me crazy.  Our van is LHD
> (import from USA) and so the passenger side mirror is vitally
> important as otherwise I end up pulling into traffic which seems to
> lose itself along the blindspot of the van.  I have replaced the
> mirror once already because the previous one wouldn't stay in
> position.  The new one lasted all of one trip and now does just the
> same, slips down and I lose all side visibility. I wonder if anyone
> has any suggestions which would overcome the problem which I guess is
> something that others have encountered?
> Thanks
> Paul
>
> Paul Clarke
> 82 Vanagon Westy
> UK based
>