The best way is to keep going out there as much as you can and paddle until you just can't paddle anymore. But I'll assume that you are like most of us and don't have time to get out as often as you would like. Swimming is a great way to get in shape. If you don't have access to a pool you can lift weights to get stronger. For surfing you'll want to target lifts that work you shoulders. Military presses and flys will give you a good work out. Check out the link below too.