If you have a hard time motivating yourself to work out alone, try joining a local sports club that plays for fun. There's indoor volleyball, water polo, basketball, indoor soccer, etc. Or you can get into the CrossFit craze, as there are now gyms all over the place dedicated to this form of exercise. Not to mention: barre workout classes, cycling classes, boxing, yoga, pilates, and so forth. With the days getting shorter and colder, you'll be more motivated to stick with it, especially if you've got fellow exercisers or teammates to hold you accountable.