Earlier this year, I had some custom frames made for prints I had bought years ago. Now granted, I had had plenty of time to think about what I wanted, but the framer was quite impressed with my quick decisions and ability to pull the art together nicely with the frame.
Here's a few tips I used in doing this:
1) Understand the look you want. In the basement, I wanted movie posters that looked like posters not ornate framed art. So simple black frames were the ticket. However, in the powder room where I have a somewhat monochromatic scheme, I wanted the frames to resemble the antiqued bronzed look of the mirror (picture above). I'm happy to report they match beautifully.
2) Pick mats that complement and accent colors in your artwork. In this example, I pick up two colors in the art - a dark gold and a chocolate brown.
3) If appropriate use a textured mat to enhance the art. In the example shown, the top mat has a texture very similar to the custom rolldown shade I have in that powder room. Between the frame matching the mirror and the mat matching the shade, it ties the room together beautifully.
4) For a more elegant framing job, a mat adds a very nice touch. And two mats are even better as it provides greater depth and contrast. I don't think that more than two mats are necessary.
5) If your poster/art is richly saturated with color, one of your mats can be white. This will help to make the colors pop out and to separate it from the other mat and the frame.
6) Look at the frame, mats, artwork, and the room as a whole. You want all of them to work together and not fight or clash.
Finally, take these tips and be decisive if you go to create your own custom frame or to order one.