Believe it or not some people actually struggle each day to put weight on. Preferably muscle- not fat, but the ability to put on weight can be just as difficult, if not more than losing weight. Inevitably when trying to build muscle mass, you will gain a little fat in the process. Of course everyone is different, so you must be willing to be your own guinea pig within reason. Experiment with macronutrients and workouts to see what brings the best results for your unique body type.
Toney Freeman is big
Here are a some methods for the hard gainer to help put on muscle with very little body fat:

1. Increase protein to 1.5g per LB of body weight

2. Include more natural fat from avocado, peanut butter, nuts, olive and flax oil. (Fats are vital for hormonal production; testosterone!) Even a small amount of saturated fat is necessary for this reason.

3. Eat 6-8 small meals every 90 minutes-3 hours throughout the day. Make sure each meal contains at least 30g of protein.

4. You may consider switching your supplemental protein sources such as shakes and bars to real food sources, such as meat, dairy, eggs, etc. Or at the very least, minimizing their consumption to twice per day. (ex: protein shake before and after weight lifting only).

5. Increase your carbohydrate intake to 1.5-2g per LB of body weight (possibly more depending on your ability to build muscle without too much fat). Ensure most carbs come from low glycemic sources such as brown rice, sweet potatoes and oatmeal. Low glycemic carbs will digest slower than high glycemic (sugar) carbs and will help you avoid gaining body fat.

6. Stay hydrated: Drink at least 1 gallon of water each day.

7. Keep weight lifting workouts intense and use challenging weights to help stimulate growth. Rep ranges should be between 6-10 reps after a warmup set of higher reps/lighter weight for each body part.

8. Get at least 8 hours of sleep each night and if possible, a 30 min. nap in the afternoon.

9. Supplementation is important for recovery and strength in the gym. Creatine, protein powder and L-Glutamine are important supplements for helping put on muscle mass.

10. Keep cardio to a minimum. Cardio depletes muscle tissue of glycogen, an important energy source for anaerobic exercise (weight lifting). The only exception would be if you are putting on too much body fat too fast.