Your body builds muscle naturally in response to strenuous activity. But if your daily routine doesn't involve challenging your muscles to grow, how do you trigger the muscle-building process? It doesn't happen overnight (as any bodybuilder can tell you) but you'll be well on your way to "bulking up" if you follow these steps consistently.

Develop a program. Decide which exercises you'll do on which days. Some exercises focus on a specific muscle (e.g. bicep curl) while other exercises call upon several muscles at once (e.g. squat). The important thing is to make sure each muscle gets a chance to rest for at least a full day before using it again. For example, you can do a full-body weight training routine every other day (with cardio in between, if you want) or you can alternate muscle groups (arms on Monday, legs on Tuesday, upper back and chest on Wednesday, etc.).

Muscles grow during rest, not during training. If you don't give a muscle adequate time to recover, you'll actually interfere with the muscle building process. When you lift weight, you're supposed to stress the muscle to the extent that it breaks down at the cellular level, resulting in increased protein synthesis, which produces thicker muscle fibers. This process begins 2-4 hours after the workout and lasts 24 hours (although some researchers suggest that muscles worked to exhaustion need 36-48 hours to rebuild). If you stress the muscle again before the process is complete, you'll break down the muscle fibers before they've had a chance to rebuild.

To build volume rather than strength, design your program around lots of repetitions (10 to 12), three to five sets, and short (30- to 90-second) rest periods between sets. Athletes looking for power and strength, on the other hand, will favor a program with reps that max out at around six and sets ranging from two to six, with a long rest period (two to five minutes) to promote better recovery between sets.

Practice good form. Learn how to do each exercise properly (full extension of movement, proper stance and posture, etc.) or else you'll not only reduce the effectiveness of the movement, but you'll also be increasing the risk of injury. To master each exercise, learn to do it slowly with light weight. Even though you might feel more accomplished when you move quickly, you're depending on momentum to do some of the work for you, instead of your muscles. Do each rep slowly, in a controlled and steady fashion.

Lift to fatigue. As mentioned earlier, in order to trigger the muscle building process, you need to stress the muscle. By the final rep of your final set, your muscle should be exhausted.

That should be the last rep you can do with perfect form. If you start losing form before then because your muscles are too tired, you're using too much weight. If you find that you can add in one or more reps in good form, you need to add on some weight.
Change your eating habits. You can't build muscle unless you give your body the proper building blocks to do so (and stop giving it junk). There are also plenty of supplements which can give you energy and aid in muscle recovery and repair, but remember, they are supplements, which means they only work in addition to a good, consistently followed exercise regimen and a proper diet.

Incorporate complex carbs and protein into your diet. Focus on lean protein like egg whites and low fat yogurt, and whole grain carbs like oatmeal and whole-wheat toast. Avoid sugary, white processed foods; they interfere with your glucose levels and immune system.

Eat small meals throughout the day. This gives your body a steady supply of fuel to build muscle. Eating in "spurts" (2-3 large meals per day) should be avoided because it hinders muscle growth during the stretches between meals.

Since creatine provides energy, take this supplement about 45 minutes before a workout. Look for products that pair creatine with carbs, as this combination increases the rate at which the creatine is absorbed by your muscles. Consuming creatine with a glass of juice will have the same effect.

Drink sports drinks during your workout. Look for drinks that contain carbs and protein. This combination reduces muscle damage and hastens recovery.

Get a carb-loaded drink or snack (1.5g of carbs for each 2.2 lbs that you weigh) within 30 minutes of your workout to stimulate an enzyme that helps the body produce glycogen.
Have a whey protein drink within 30 minutes of your workout to help your body repair and rebuild lean muscle tissue.

Change your routine every four to six weeks. As your body adapts to stress, you'll hit a plateau where the benefits of weight training will begin to diminish. The only way to prevent this from happening is to change things up, such as by increasing weight and changing exercises.

While doing cardio will help burn fat so your emerging musculature is more evident, doing it for more than 90 minutes will favor a lean physique, rather than bulk.

Most bodybuilders greatly limit their cardio while they're "bulking" (building muscle), then add more cardio when they're "cutting" (shedding fat).

Some people are genetically predisposed to build muscle easily. Other people may need to experiment with different eating habits and training routines to find what works for them. Most women are unable to build large muscles like men, due to differences in hormone production, but they can still build muscle; it just won't be as "bulky."

Get enough sleep. Sleep is a critical element of rest. Avoid caffeine and alcohol for deeper sleep.

Ask someone to "spot" you, so that you can safely push your muscles to the point where they fail. Your spotter can then help you with the weight just enough so that you can complete the motion. Having a spotter for exercises like bench press is normal and expected, so don't be shy to ask someone. And if you can't find a spotter, don't lift to failure on any exercise where a dropped weight could injure you.

Stay hydrated!

Many people you will encounter in a gym have misguided ideas about physiology and proper athletic programming. There's a lot of "gym mythology" floating around. Take others' suggestions with a grain of salt, and always ask for sources (like the ones provided below).