Building up muscle mass is not very difficult for most builders, especially if they work out correctly and hard enough, and consume a lot of healthy food. But mass as a whole should not be mistaken for pure muscle mass! A lot of builders who emphasize too strongly on food, despite its being healthy, store a thick layer of fat under their muscles. Building up pure muscle mass is an entirely different thing! Professionals know how to add muscle mass without fat which makes you look plump and forces you to buy two-size larger jeans!
Three of the champions of nowadays, NASSER EL SOMBATI, MIKE FRANCOIS AND KEVIN LEVRONE, have been discussing how to get your muscles separated and remain firm without losing muscle mass. NASSER EL SOMBATI:

Since he settled in California, Nasser has quickly fit into the elite of bodybuilders.  He won “The Night of Champions”and “Huston pro” and came out third in “Mr. Olympia”

"To lower your fats level you must increase the intake of proteins and cut down on carbohydrates. At least half of the diet should consist of high-quality protein like turkey, chicken, tuna fish and protein drinks, 35% of complex carbohydrates like vegetables, carrots, cabbage, etc., and 15% of fats. Potatoes are good but limit their intake! I have found out that when I take in more carbohydrates I cannot burn enough fats – the muscles use carbohydrates for energy, instead of the fats. I have achieved the best fat-burning results when I maintain a lower intake of carbohydrates for a few days, then increase it for a day and then decrease it again. My body is unable to adjust so it intensifies metabolism.  
Now I don’t take in carbohydrates after 8 p.m. because I found out they make me feel hungrier. If I feel hungry, I have a protein drink. Many builders are afraid to work for muscle separation because they think they will lose muscle mass. If you do it correctly, what you lose straightaway is water. After you lose the excess water the burning of fats begins. The key to success is to begin the diet slowly, allowing enough time – I suggest 16 weeks – which will allow your body to slowly lose fat. This peak of your separation program should be reached once a year. More than once is too hard on your body. I did it twice in 1995 but I do not advise anybody to do the same.
When starting the separation cycle you must gradually exclude the dairy products, pastries and sugar. Do aerobics during the first three weeks – three times a week until you gradually reach 7 times. At the peak hold this situation for 1 or 2 weeks. Your body will start to look fantastic, your skin will get thinner and show the splitting and separation between the muscles and you will look more muscular as a whole. Once you have lost excess fat you will find out it has become more difficult to put on fats again. This is the greatest benefit from working for separation. When you lose 6 or 8 kilos of fat you will look bigger and more massive than before starting your diet!