If you're going to want to gain muscle, you're going to need testosterone. There are many natural ways of boosting testosterone to increase your gains, but there are also some things you do not want to do if you're trying to gain muscle.
First major thing is to keep your workouts short and sweet. Ideally under 2 minutes of rest per set, and no more than 45 minutes per session. After 45 minutes of hard training the body begins to produce cortisol, which is a catabolic, or muscle destroying hormone. This will mitigate any muscle gains you're trying to put on if you're constantly in a state of cardiovascular stress. Keep the cardio high intensity for short intervals and you'll see the same effects of long, slow cardio.. without the diminished muscle size!
Another major component to keeping your hormones in balance is your diet. Healthy fats, like those found in Olive Oil, Avocados, Lean Meats, and Fish are naturally testosterone boosting. High Glycemic carbs, or "bad carbs" as you will commonly hear them called (white breads, rice, sugars, potatoes) cause your body to have a cortisol response, which has a negative impact on muscle gains. Keep your diet clean with low glycemic, high nutrition value carbohydrates, healthy fats, and lean proteins.
These are just a couple tips to help you maximize your muscle gains, I'll be writing more about hormones and their effects on training in the future. As always, stay strong and informed!