Overweight people. Being overweight, but especially obesity increases your risk of developing diabetes. To decrease your risk, shed the extra weight and then maintain a normal weight. Eat lots of fruits and vegetables, lean meats, and minimal wheat and other starches. Exercise daily and keep yourself in shape. Diabetes can still happen, but you've lessened the chances tremendously.
People older than 45. Apart from the obvious such as lack of exercise, obesity, and family history, age plays a important factor. According to the cdc diabetes incidence, if you take 1000 people age18-44 about 5 will be diagnosed with diabetes. If you take 1000 people age 45-79, 12 will be diagnosed with diabetes. Look at the steep yellow line in graph. http://www.cdc.gov/diabetes/statistics/incidence/fig3.htm.