You know you want it! More muscle that is! Now before all the women tune out as I can already hear them crying: "But I don't want to get too big." Trust me, you can cut back just before that happens. Muscle is a very metabolically active tissue, so you will burn more calories at rest by carrying more muscle. Just like a big engine burns more fuel, adding some muscle will allow you to eat more!
Add in that many suffer a loss of muscle as they age (sarcopenia) which leads to a loss in function of simple daily tasks (1). All in all, great reasons to add some lean muscle, so let's get right to it and break down 5 simple ways to get 'r done.
1) Protein
Protein is the building block for muscle, in addition to many other functions in the body. The main area of storage for protein is in muscle tissue itself. Consuming protein serves 2 main effects: 1) starting protein synthesis 2) supplying raw materials (3).
Think of this like a light switch. It takes the act of pushing up the light switch to turn the light on. This is similar to starting the process of protein synthesis, where the body will take proteins that are circulating and stuff them into muscle tissue.
The light switch also needs to see electricity going to it, otherwise nothing happens. The electricity is like the raw materials (amino acids) and are the actual building blocks of muscle. No light switch or no electricity = nothing.
A similar analogy is brick layers building you a house. You need them to start working (start protein synthesis) and they need raw materials too (amino acids in this case). Research has shown that essential amino acids and lifting weights will both stimulate human muscle protein synthesis (4).
How much protein?
The general amount that is batted around the ole interwebz is about 1 gram per pound of body weight. If you are a 200 lb male, that is about 200 grams of protein per day. While there is not one particular study that used this exact amount, there are data to show that this is a good starting point and probably on the high end (9).
While the debate as to how much protein you need will be debated for decades, most are looking at protein intake for fat loss. In this case, calories will be lower to selectively lose fat, but not muscle. I'm sure you all have seen someone who goes on an extremely low caloric diet and just looks like a smaller version of their old self. They may weigh less, but they don't LOOK any better. We want to keep as much muscle as possible while getting rid of fat. The amount of protein to accomplish this body recomposition is around 0.75 grams per pound of body weight at a minimum (2, 8, 10). If you weigh 140 lbs, that is about 100 grams of protein per day.
2) Overload
Tissue responds to load, so work to increase more overload via volume, density (volume/time) and intensity (weight on the bar). Many people in the gym tend to only focus on the load on the bar (intensity). While this is good, we all know that you can't add weight to the bar every session or every week forever. If we could, people pulling double to triple bodyweight deadlifts and bench pressing 400 pounds would be as common as treadmills in every gym.
The key is PROGRESSIVE overload, which has 3 main components
1) More volume (weight x sets x reps)
2) Better density (volume / time) 
3) More intensity (weight lifted)
This may not happen all in the same session, but EACH one of those is a form of overload.
Your body will positively respond to it by adding muscle and strength, just in case you do it again! The typical progression that I see for an exercise is more volume, more volume, more volume, better density, and more intensity.
You will probably be different, but that is a rough starting point. The key is to work to accumulate more of each over time.
3) Essential Fatty Acids
Fatty acids play many, many roles in the body. Every cell membrane uses them as building blocks. Newer research also indicates the essential fatty acids like omega-3 (and their downstream components, EPA and DHA, commonly known as fish oils) may be anabolic and promote muscle growth (11). Smith, G.I et al. (11) and friends, demonstrated that omega-3 fatty acids stimulate muscle protein synthesis in older adults. Fish oil is a great source of essential fats. Many people are deficient in these crucial fats and, as the name implies, they are essential and cannot be made in your body. Pretty simple to do also!
4) Testosterone
Unless you have been living under a rock, you have heard about testosterone and muscle growth. Testosterone has been shown to increase muscle hypertrophy, lean body mass, strength, improve energy, increase libido, and even reduce your risk of a heart attack (5, 6). Sign me up!
Testosterone can also increase the number of satellite cells too! (7) Satellite cells "hang out" at the end of the muscle fibers and serve like little repairmen that spring to action when the muscle fiber is damaged from a heavy weight training session. Research by Tamb et al. (12), showed an increase in the testosterone concentration in 320 older subjects after taking a standardized water-soluble extract of Eurycoma longifolia (Long Jack) for one month. It also correlated to a reduction in symptoms seen in aging males. Increasing testosterone is another way to add some more muscle.
5) Exercise Selection
Tissue (muscle) responds directly to overload.
Want bigger shoulders? Pick an exercise that stimulates the shoulder muscles. Score one for the old time bodybuilders. If you are cramped for time, I recommend you look for more "bang for your buck" compound lifts like bench press, squats, overhead press, deadlifts, pull ups, etc. This allows you to stimulate a whole bunch of muscle in a very short period of time. These exercises enable you to use heavier weights also in addition to being fun! Most everyone can deadlift more than they can preacher curl!
Summary
Now you are armed with 5 tips on how to add some more metabolically-active muscle tissue to your frame. Muscle always looks much better than fat, so add some today!
