Here's a method that works well for me. May take some time to read and understand for some without the aid of pictures, but read carefully as I try to simplify this as much as possible.
I drill a small hole with a pin vise and a micro sized drill bit ( available at shops that sell model trains) into the mirror at about 1 mm deep. Be careful not to drill all the way through! Then I cut small diameter styrene rod that fits into the drilled hole and glue in place. You should leave about 2mm excess for use as guide pins. When drilling make note of the angle you want the mirror at, otherwise you'll get lopsided results.
With the rod now in place. Line up to where you want it permanently affixed to the body and place a mark where you want to drill. Again, make note of the angle you're drilling at. You don't need to be perfect about the angle as you can bend the styrene rod a bit to conform.
The last step is insert the side mirror and glue with white glue from the inside. I say use white glue because if your model is painted, you don't run the chance of ruining the paint job if you get excess on the painted areas. Trim off stubs if necessary.
DJMR2-hope this helps good luck! How's your Gundam stuff coming along?
-Walter