Liver spots, also known as "age spots" or "solar lentigo" are best prevented by sunscreen and avoiding the exposure of sun to those areas.
For cosmetic purposes, these dark skin spots can be treated by a dermatologist (I believe some of the products used contain alpha hydroxy acids or retinoids). Laser treatment may also be considered.
The Mayo Clinic has a full page, describing age spot treatments: http://www.mayoclinic.com/health/age-spots/DS00912/DSECTION=treatments-and-drugs, including:
- Laser Treatment
- Prescription medications for lightening spots
- Chemical Peel