Consider the style of the art. The frame you choose should complement the piece within. An impressionist-style painting will look ill at ease in a contemporary stainless steel frame, while a spare, modern work won't jibe with a richly detailed gilt one. Select materials and a profile that feel appropriate to what you're framing.
Consider the style of the room. The framing you choose doesn't have to match your decor exactly, but it should at least feel as though the piece hangs in the right house. The traditional frames used for this pair of paintings complement the room's classic, understated style.
Match the size of the frame to the size of the picture. A thick, chunky frame can overwhelm a diminutive piece of art, and a skimpy one can fade away next to a strong, overscale painting or photo. Choose one that will balance the work it surrounds.
Decide whether to use mats. Not all pieces need them, and it's largely a matter of personal preference. Hold mats and frame moldings against the work in question to determine how you like it best. Art with a clean background, such as the shell prints in this bedroom, works especially well without a mat.
Choose a mat that contrasts with the color of the art. Generally, you can't go wrong with classic white or cream (trendy colors are a big no-no, unless you don't mind swapping them out every few years). But black and white photos, line drawings and pencil sketches can look striking with a surround of black. Just be sure there's a strong contrast between the art and the mat — a murky piece will disappear in a field of darkness.
Select the type of glass you want. If your piece is especially fine or sentimental, you might choose to invest in museum glass, treated to block UV rays that can fade colors over time. It's expensive, but it can pay off in the long run. Regular glass is a lower-cost alternative, but it can be too heavy for a very large piece. In those instances, lightweight acrylic is often the best bet.
Make sure mats are wide enough. A mat that's too skinny won't set off the piece it surrounds. Here's a handy rule of thumb: For the best effect, make the mat at least twice as wide as the frame. You can go wider, but be sure the art doesn't get lost.