How should I maintain or improve the condition of my liver?

Diet, lifestyle. Eating a healthy diet and avoiding alcohol, tobacco, and drugs is the best way. By drugs i mean both illegal drugs and over the counter ones like acetaminophen.
Liver Health. Avoid liver damaging substances - alcohol, street drugs, Acetaminophen (tylenol) more than 4000mg a day. Many medications and supplements can cause liver damage, so review that with your doctor. If you are overweight, lose weight. If you have a viral infection (hep b or hep c) get it treated! eat a heart-healthy diet. Exercise regularly.