Well. That's a pretty wide-open question, and the slightly flippant answer would be whatever looks good.
As with anything, it all depends on the look you're after, but I'd say don't be afraid to go all-out. After all it's probably the most expensive piece of paper you'll ever own.
A design you almost can't go wrong with is a lovely frame, neutral mat and a fillet. A black mat is always elegant, or you could go with something similar to the color of the paper. And it goes without saying that a fabric or fabric-wrapped mat is the cat's meow. Care to chime in here Baer? Of course for better preservation you should put another mat below the fillet, and that one could be the same color as the top, or an accent color.
Now some people like to do the school colors in the mats. This is just my opinion but I think that's kind of tacky. I mean it's great if you're a 22 year old frat boy, but it may look a little silly when you're 30 and have a nice office. So I'd suggest in those cases a neutral mat on top and then a small accent of the school colors with a couple bottom mats. If it's a light colored top mat I'd put a v-groove in it. Or the school colors could be incorporated with some nice French lines.
And again, purely my opinion, but I don't care to frame the tassel and other doodads with the diploma. It's ok for high school, but I think it cheapens the degree. You could always frame the tassel, some photos and the announcement in a second frame and get another job out of it.
Whatever you do don't put a "simple black frame" or a "certificate" frame on it. Boo hiss. I don't know where customers got the idea that diplomas are supposed to go in little black frames. But absolutely do use conservation glass or museum glass, and despite what some may tell you, don't drymount it.
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