Alternator belt won't stay tightened . . . .

[Ice Cat ^. .^ ~]

I am wondering whether anyone has had the problem with an alternator
belt that just won't stay tightened.
First it starts to squeak and slip, and then shows not enough juice
being returned to the battery when I drive.
Each time my husband tightens it, it stays for less and less time . .
. ???  Now it will not hold for even the first drive.    We put on a
new alternator belt which we purchased from the dealership after the
last one which looked great didn't stay tightened.

Of course we don't want to over-tighten it and wreck a bearing, but
is there a trick to tightening it without tightening too much or too
little ?   Fay

'89 Audi 200 Turbo (No Q)

  "A computer once beat me at chess, but it was no match for me at
kick boxing."