Dave wrote:
Planning to mount a mirror to the door of a wall cabinet. The door
will probably either be wood or some type of particle board.

Wondering what the best way to do this is. The first thing I thought
of is to glue it. But sometimes you can see the glue through the
mirror.

Any recommendations as to either the right type of adhesive which
wouldn't cause this problem (or how to prevent it) or another way to
mount the mirror? I'm concerned that anything other than glue might
not be rigid enough since the door will obviously be moving.

This is the kosher stuff to use to glue mirrors. Definitely don't
attempt to use anything not intended for mirrors:

http://tinyurl.com/22yd2v
http://www.screwfix.com/prods/42317/Sealants-Adhesives/Adhesives/Grab-Adhesives/Mirror-Adhesives/Mirror-Bonding-Adhesive-310ml

There's no way you'll see this through the glass (unless you've got a
knackered old mirror with the silvering flaking off).

My only reservation would be whether it's OK for a slightly flexing
surface like a cupboard door - I'm not sure about that (as in, I don't
know!).

Alternatives would be to drill holes and use domed-capped mirror screws.
You can also get little metal angles brackets which cover the corners
(can't find a picture online) but they aren't very attractive.

David