1. Make sure the frame doesn't draw attention away from the painting. It needs to complement the painting.
Don't imagine that because a painting is small that the frame also needs to be small. Frequently a small painting is enhanced by a wide frame which could actually overpower a larger painting. Generally when framing paintings, you can say the larger the painting, the narrower the frame required.
Also an ornate frame with a busy painting can make the viewer feel uncomfortable.

2. The frame needs to suit the style of your home. With most old homes you can successfully have both old style and modern frames but it's much harder to hang a painting successfully in a modern home if it has an older style frame.

3. Gilt tells the viewer you have respect for the painting. If framing paintings in gold, a simple frame can suit any style home.

4. It's not necessary to frame art on canvas if the artist has painted the sides but you need to make sure the stretched frame is sufficiently reinforced. If you wish to have a painting without a frame, make sure that not only has the artist painted the sides, but that there are no tacks or staples showing.
Framing paintings does give structural support.

5. When framing large contemporary paintings, a narrow floater frame measuring half to one centimetre is often all that is required. Some framing can be understated and simply give the impression of an unframed painting.

It's important and exciting to experiment. I hate rules so only use the above tips as a guideline for framing paintings.