What is the best way for me to keep physically fit for diving?

The best way to keep physically fit for diving is by following a general fitness plan.  Ensure that you exercise your cardiovascular system so that you have a strong heart and lungs and be sure to improve muscular strength.  There are so many ways to accomplish each of these tasks that you can choose what is best for you.  Running, swimming, skiing, weight lifting, aerobics, yoga; these and more will help you keep fit enough for scuba diving.