Summarize this article:

Your body has the ability to adapt quickly, so if you don’t alter your exercise routine occasionally, you will hit a plateau. Once a week, change up your routine. You can increase or decrease the number of reps or sets, or simply change the order in which you normally do your routines. When you run, bike, swim and do other cardio exercises, you’re using up energy that could be instead funneled toward muscle growth. Limit your cardio to 20 to 30 minutes daily when you’re trying to gain weight. You can also choose lower-intensity exercises like walking, hiking or short bike rides in flat areas. There’s another method for gaining weight fast: eating whatever you want and moving as little as possible. However, gaining weight this way isn’t likely to give you the appearance you want, and it will make your body weaker instead of stronger. Putting in the hard work to gain weight by building muscle will result in improved health and a built appearance. In your eagerness to gain weight as soon as possible, you might be putting a lot of stress on your body. You shouldn’t feel exhausted and sore all the time. In fact, your improved diet and workout routine should help you feel more virile than ever before. If your body seems to be telling you that something’s wrong, listen to it.  Consider hiring a personal trainer. Within a few sessions, you'll have a great idea of scheduling, form, intensity, and duration of workouts and excellent insight into tweaking your diet. Ask your physician before taking any supplements, and see a doctor right away if you experience an injury during a workout.

Summary:
Don’t get stuck in a rut. Limit cardio sessions. Move around instead of being sedentary. Don’t ignore signs you’re gaining too fast or training too hard.