Skimping on Protein

Women lose about 5 pounds of muscle every decade (and gain an average of 15 pounds of fat), Protein helps you build lean muscle, so make sure you're getting enough: If you're in your 30s, aim for 50 to 56 grams daily; in your 40s, you need 56 to 62 grams daily. Good sources include tuna and chicken breast (6 oz of either will give you 45 grams of protein).