Depending on your budget, start with new levers such as Cane Creek or Tectros. They are comfortable, not too expensive and provide great leverage for better stopping power. The only problem is you will need to unwrap and re-wrap your bars to make the change. New brake shoes will also help. Koolstops are considered by many to work very well. Try these two things before buying new calipers. Looks as if you have centerpull calipers. They should stop you as well as any.