I have been doing some experiments with scrap wood on techniques to get
tight miter joints prior to doing the real thing on my project. This
technique worked well for me so I thought I would pass it on:

"How To Master The Radial Saw" by Wally Kunkel. This is a short version
(without diagrams unfortunately).

1. Get a roll of filament reinforced packaging tape.
2. Miter the corners accurately, leaving about a 1/64" flat area at the tips
(I did not do this part)
3. Lay the pieces down on a flat table outside up with the flats abutting,
and strap them in this position using pieces of tape about 10" long running
perpendicular to the joint.
4. Turn the pieces over and put a fairly thin line of glue in the bottom of
the v groove formed by the two pieces.
5. Fold the pieces to form the right angle between the pieces.
6. Strap in this position using more tape. If you had the right amount of
glue in the groove, the folding operation will spread it evenly in the
joint. To get a tight joint, accurate miters are essential. You may need to
adjust your saw.

Dick Snyder