I tried tightening up my existing mirrors, but it didn't help.  The
plastic was cracked and no amount of tightening would keep them in place.
Gluing them in place with Locktite didn't work either, so I broke down and
bought new mirrors.  They'll cost you $25-40 each depending on where you
get them, more if you buy convex ones.  (I prefer flat mirrors in my Westy
because I can use them to judge distance when backing.  There's not enough
visibility through the back window to help.  I do add 2" diameter stick-on
convex mirrors to fill in the blind spots.)

I still had to tighten up the tension nut on the bottom of the new mirror
(I've only installed the driver's side one, so far) but it's held position
while driving into some pretty stiff headwinds.

My only complaint is the quality of the driver's side mirror I got is a
bit lacking -- the edges of the mirror are actually a bit concave, so
there are distortions in the image.  It's usable, though.  I got that one
from Parts Place.  I've ordered the passenger side one from Bus Depot in
hopes it'll be better.

David Brodbeck, N8SRE
'82 Diesel Westfalia
'94 Honda Civic Si