Meat
Lean meat is an excellent form of protein, and protein plays a key role in regulating how much you eat, as it satisfies your appetite and stops you feeling hungry for longer. Proteins take a long time to be broken down by the body, so they help you avoid spikes in your blood sugar - meaning you won't be reaching for the biscuits.
Always choose the leanest cuts of meat (or trim off excess fat) and don't eat more than 70g per day. You should avoid processed meats as they have been linked with a higher risk of colon cancer.
See also: The 10 worst foods you can eat