Stay within the limits of your training and experience
Divers must adhere to certain limits based on their certification. Sometimes you don’t understand why not to descend those few extra meters, but these limits were established for good reason. Exceeding your certification level can put you in a stressful environment that you’re not trained to handle. PADI also calculated these limits based on gas, depth, and physiological considerations. If you want to go deeper, think about advanced training. 

Stay in good physical condition
You do not need to visit the gym every day, but it would be nice if you could take 30 steps with all your scuba gear and tank on without having to sit down and catch your breath. Although diving may not be as physical as some other sports activity, maintaining good physical condition is nonetheless important both on land and underwater, when situations like unexpected current can put your cardiovascular skills to the test.

Keep your equipment in good working order
Your scuba equipment is a highly technical gear that is responsible for keeping you alive, so take the extra time and spend the extra money to make sure it is functioning properly: service your equipment in accordance with the manufacturer’s recommendations, check it before every dive and clean (freshen) thoroughly after.

Master neutral buoyancy
Mastering neutral buoyancy not only makes you look professional, it also provides for longer bottom times. It saves your air, saves your energy, putting less stress on your body. And these can help to do your dive safer.

Have sufficient rest before a dive
Getting a good night’s sleep results in a healthy rested body. This, in turn, will help you resist the effects of harsh conditions, such as cold water. Being tired also slows your mind and body down, and makes you work harder than you normally would.

Stay hydrated and well nourished
Have a good breakfast, but leave a couple of hours after a meal before entering the water. Also, drink a lot of water before, between and after your dives. Just remember to use the restroom before you get in your wetsuit!

And finally: have fun and stay positive!