I forgot I wanted to share this tip I read online. (I don't think it was here, if I am stealing your tip I'll gladly give you credit)
To give your frame a more proffesional look get some kraft paper (like paper grocery bags or other brown paper) attach it to the back of your frame with double sided tape or hot glue. Then get the paper wet, not ruin your project wet, just moist. As the paper dries it will shrink to a nice tight fit over the back, making your framing job look even better.
This works with shop-made frames as well as store-bought frames.