You can enjoy fast food now and then if you order wisely. If the restaurant offers several sandwich sizes, pick the smallest. Bypass hamburgers with two or three beef patties, which can pack more than 900 calories. Instead, choose a regular- or children's-sized hamburger, which has about 250 calories. Also, skip the large serving of french fries and ask for a small serving instead. This switch alone saves 200 calories or more.