Living in Arizona, I don't get to dive as often as I'd like to. Therefore, I focus on other hobbies and activities to stay in shape.
Diet is probably the most important because it doesn't matter how much you workout, if you eat like crap you'll never reach your goals. Otherwise I just try to stay active by rock climbing, mountain biking, and hiking. Increasing your cardio will help to lower your air consumption and the less body fat the less weight you need when diving. It all plays together, make small and traceable goals and just take it day by day.