1. Keep your weight within normal limits

2. Quit smoking (and also stop using other harmful substances)

3. Limit your coffee and alcohol intake

4. Limit the use of your mobile phone

5. Take care of your diet

6. Stop taking anabolic steroids

7. Take care of the temperature of your testicles

8. Get tested for genital infections

9. Avoid injuries at your genitals

10. Treat your varicose veins

11. Take your age into account

12. Protect yourself from harmful environmental factors

13. Take note of your medications

For more information, read this