I've been down that road. It comes down to two issues: are you really cutting 45 degrees, and is each board exactly the same length (at least the boards opposite to each other). Often the latter is the problem. The best way to cut is with a power miter box (with the proper fine blade), equipped with a positive board length stop (not sure of the exact term). Using this set-up, they come out square every time. Trying to cut using a table saw can be tricky without the right accessories. There are tools made specifically for cutting picture frames that get great edges - you might be able to rent one.