Step 1: A quick and easy solution to miter joint gaps
Step 2: The final fit
If your gap is large, bring the two members of the joint together and run the saw between them again. In most cases, you will not create a fit problem for the other joints by removing a tiny bit more material at this corner.
If you are working at installing molding in a house, be aware that the corners on many walls will often be a bit greater or lesser than a perfect 90 degrees. Take a couple of pieces of 1 x 2 two or three feet long that are very straight. Fasten them together at one of their ends with a bolt and wing nut. Use this as an angle finder to check the actual angles of your corners. Measure the opening in the angle finder with a protractor. Adjust your miter saw accordingly. If there are still inaccuracies in your cuts, you will have less material to remove with a dovetail saw in order to get rid of any gapping.
This idea originally came from a high school woodworking textbook published in 1910. I believe it was called "Woodworking for Secondary Schools."