Avoid alcohol, nicotine and caffeine
Alcohol may make you fall asleep easier for the moment, but impairs the quality of your sleep and makes you sleep shallower and more restless. People who intake nicotine have more difficulty falling asleep. Caffeine has different effects on different individuals; some can handle caffeine late at night without it affecting their sleep quality.
Decrease stress
This is the most important point but perhaps also the most difficult to overcome. No matter how much you try to fix the sleep problems, one fact remains: if you are stressed, your sleep will be affected. Stress can be physical, psychological, temporary or long-term. Too little sleep can also lead to stress and the whole thing becomes a vicious spiral. Some simple measures to reduce stress can be exercise, mindfulness and relaxation exercises or seeking help.