In terms of food, red meat is by far the best source. The iron in plant foods is bound to compounds such as phytates which interfere with absorption. When your iron is low the body is able to increase absorption. A good way to increase your iron is to take a daily multivitamin for women (it should have 18mg of iron) along with a 500mg chewable Vitamin C. The Vitamin C increases the absorption of the iron dramatically. Most of my patients achieve normal iron levels within 2-3 months without needing the huge doses of iron that are hard on the gut.
Not sure of foods but I take an iron supplement as I've always had low iron. Also I believe spinach is an excellent source of iron.