The following 3 habits will result in weight gain faster than you can say 'anti-fitness habits.' Break these habits quickly.

1. Drinking Calories. Soda pop, coffee drinks, smoothies and fruit juices are filled with calories. Since liquids won't give you a 'full' feeling, these drinks slide by without notice of how many extra calories you're getting. Stick with water to avoid extra calories.
2. Not Exercising. Skipping the gym day after day, week after week, and month after month will lead to muscle loss and a reduced metabolism. Exercise 3-5 days per week to keep your muscles strong and metabolism high.
3. Eating a Heavy Dinner. Eating a large heavy dinner sets you up for weight gain. Instead of eating the majority of your calories late in the day, spread it out over the entire day.