If you have a matted piece that is not yet framed, avoid touching it unless you are wearing gloves. Oil in the skin can leave spots that are difficult to remove. If a spot shows up, try to blot it with a dry cleaning pad.
Once a mat is behind glass or acrylic, it is still susceptible to other forms of damage. Ultra-violet light rays can accelerate the fading of many mats. Use conservation grade glass or acrylic to protect the art and mat as much as possible. Excessive moisture and temperature changes can cause water stains, mold and buckling. Inspect your framed art regularly to avoid irreparable damage.