When deciding to reframe a piece of art (or not!) remember that your original frame may have value as well. There are so many types of frames, I am just going to discuss the two I come across most as an appraiser; 19th century and 20th century frames. Consider the following:
In the first example below, the frame was made in the late 20th century. Notice the silver finish. Do you see the small black lines that run through the silver? This is evidence that a machine finished the piece. The frame also lacks a warm patina that is indicitive of an older surface.
In the second example, the frame was completed in the 1850s. Notice the sand-textured cove design. The ornamentation was hand-stenciled onto the surface. At the top of the frame you will see a small 1/4" vertial line. This is referred to as a leaf line. It usually indicates that the surface of the frame was hand gilded with gold or metallic leaf.
"What about reproductions?" you ask. Good question. Reproductions can be tricky. Although the gilding often looks newer, lacking the warm, aged surface of older frames, some are made extremely well and will fool the best appraisers. One of the most important things you can do is inspect the back of the frame. The wood should tell you everything you need to know. If there is paper covering the back of the piece, peel it back a little and take a look. If the wood looks light (as if it were just brought home from the lumber store) then it is probably a new frame. However, if the wood is dark, uneven and perhaps even a bit holey then it is probably older.
So many antique reproductions are made in China and Mexico today. The next time you go to a big box store check out the large posters with the decorative frames. These frames are hollow. They are not made the same way as the older frames which tend to be much heavier.
Condition: Although design is sometimes more important than condition, usually condition is a major factor. If the surface of an old frame has been regilded, repaired, or restored then it may be worth less. A good appraiser should usually (but not always) be able to tell by the appearance of the joints.
Frames in Relation to the Work of Art:
The more important and valuable the work of art, the more expensive the frame will become. Although frames do have value (sometimes a great deal of value) on their own, they are made more valuable if they are original to the piece (especially if it is an important piece) or are of the same period as the piece.