Before you get into the gym, make sure your nutrition and supplementation are in order. Are you eating enough calories? Are you eating too few? Your body needs enough fuel to get through two workouts each day, and enough protein, carbohydrates, and fat to help your muscles recover and grow.
Being lean and fit takes dedication to your training, nutrition, and supplementation. You can't rely on one to make a difference in your physique; you need all three working together. Do your best in the gym, but also do your best when you leave it.