Liver spots or age spots are pretty common. They are mainly related to the environment, genetics, nutrition as well as other factors such as sun exposure. For example, fair skinned people tend to show age spots or liver spots more than a darker skinner person.
Medline Plus.com indicates that preventing sunburn, using sunscreen and wearing protective clothing while exposed to the sun is the best way to prevent sun exposure. Good nutrition and fluid intake in another helpful prevention tip. One other thing that helps is keeping your skin moisturized with lotions that are not full of perfumes.
For the full article relating to Medline Plus click here http://www.nlm.nih.gov/medlineplus/ency/article/004014.htm.
The American Academy of Dermatology also provides information on how to treat liver spots once you have them. You can find that information here http://www.skincarephysicians.com/agingskinnet/basicfacts.html.
Hope this helps.