Hi Jason
For a frame to be square and the angles tight....
*Every 2 opposite boards must be exactly at the same length
*The miters must be exactly 45° or, the sum of two angles must be 90°
*The miter itself must be cut vertically at 90°
To overcome small deviations, the best is to cut the miters on a table saw with a sled.
Use the Left/Right method and instead of putting the stop 4' behind, put the stop at the front.
I think that you will see the method on the next issue of WOOD Magazine but in the mean time have a look at my very first post on SMC...
http://www.sawmillcreek.org/showthread.php?t=36590
Regards
niki