If you image is on paper there are different ways to attach it to the backing board:
a: Museum mounting- This is really the best way to mount a high quality image. The framer uses a linen tape (acid-free) and creates "hinges" or gently tapes around the edges. The drawback is that over time you will probably see some wavyness (because the paper is not fully mounted to the board) which has never really bothered me but does bother some.
b: Dry Mount- A thin piece of tissue is placed between your image and the mounting board and then they are pressed together in a dry mount press. The tissue melts and adheres to your image and the board binding them together while the press flattens everything out. New technologies have made this process better (there is now heat release tissue) but all of that goo is not that great for valuable images.
c. Spray mount- This is the stuff you can buy in the craft store but it is often used for framing. The spray is in an aerosol can and the material is definitely not good for your art and almost impossible to remove without a professional restorer.
2. Backing Board and Matboard
Have you ever noticed a brown line around your image where the mat was once located or has your image developed a brown/yellow color or strange brown spots? All of these things indicate that your image has absorbed acid. You can fix this through restoration but you can avoid future damage by following the advice below.
Make sure you do not use, cardboard, posterboard, wood, or styrofoam. Acid-free matboard or acid free foamcore is the best option and it will help preserve your image for many years.
If your image is anything other than a poster you do not want the glass to touch it. There are a variety of reasons for this. If you have a photograph and a drop of water gets under the surface of the glass (from cleaning) it will spread over the entire surface of the photo. If the image is a pastel, the glass will smear the image etc.
To keep glass away from the image use matboard or spacers. Matboard is usually only 1/16" thick. This thickness will not be enough to protect the art. To compensate, you will need to use a double mat to ensure proper spacing or a thicker mat (they are available at a higher price). The other option is a spacer. Placed inside the lip of the frame, a spacer acts as a little shelf for the glass, keeping it away from your artwork.
Remember, a good appraiser always checks for condition. Quality framing is a tool you can use to ensure your artwork retains its value.