What's the best glue to use for gluing wood moulding to plaster walls?
 What else should I know about this process?

As noted by knowitall22, experts suggest using either carpenter's wood
glue or Liquid Nails to install moulding. Here are some links to
detailed instructions on how to install various mouldings.

Istallation instructions for Crown Molding and Cornice: 
Predrill and countersink holes for screws near top and wall parts;
approximately one screw for every ft. of cornice/crown molding
Apply liquid nail glue on the back of crown molding near ceiling and
wall edges, but not too close
Put crown molding up against the wall and drill the screws in. wipe
off excess glue of the edges if necessary
http://vytasornamental.net/vcorniceM.htm

Detailed, illustrated instructions for installing moulding are on this
site:
http://www.jeffgreefwoodworking.com/pnc/moldings/

Instructions for baseboard molding:
http://www.thisoldhouse.com/toh/homehandbook/article/0,13422,216118-4,00.html

And for crown molding
http://www.thisoldhouse.com/toh/printthis/0,13588,214981,00.html

Crown Molding and plaster walls 
Can anyone tell me the best way to attach crown molding to plaster
walls...?
I had problems finding the studs and keeping the molding straight. I
made angle brackets,2X4 with the correct wall angle , attached them to
the studs, and then just nailed the Crown to the brackets, Much easier
to deal with. At the corners or joints, I liquid nailed the brackets
up and this gave me a good nailer even without studs. Crown Molding is
not for the faint hearted, but this made it much more manageable.
Another idea: A stud finder with a 2 inch nail finder.
Still another thought: They are attached the same way to drywall
walls. You have to locate the studs and ceiling joists and fasten the
molding to them.
http://www.handymanusa.com/questions/trimmoldingq.html

It is the application of the glue that will create a firm bond
between the molding and the surface to which it is to be adhered.
It is only necessary to apply the glue to the outer edges
of the molding. This will give you strong and effcient adhesion
Glues vary throughout the world so we are unable to
recommend a specfic brand name glue.
However in general terms any 'quick bond' glue
will do the job!
http://www.moldco.com/usage.htm


search strategy: wood molding plaster glue/adhesive; wood trim plaster
adhesive

Nellie Bly
Google Answers Researcher

The strength of any bond is dependent on whatever substrate is
weakest. Just another way of saying the old cliche: The chain is as
strong as its weakest link. In this case, it would be the plaster.
Since you are bonding wood moulding, presumably decorative, there
should be no unusual stresses upon the moulding, therefore many
adhesives are suitable.
Were it myself, I would use PVA glue (Elmer's Glue All or equal) or
Liquid Nails, which has enough open time to manipulate the work.
Another powerful bonding agent is silicone adhesive, which adheres to
almost everything. In any event, make certain the plaster is sound,
remove all dust and dirt from it before you begin.