Don't buy planes with bad fuel effieciency (Concorde or old Russian aircraft)
Make sure you buy fuel tank and maintanance depot at your hub, this will increase your income and decrease your expenses.
Try to form a closed flight network in order to maximize the efficiency of offices.
Start hubs that have good stats but are not too popular (e.g. Doha, Phuket, Tampa etc.)
Identify cities that have low competition and build routes there.
Make sure that you get as many slots between two cities as your aircraft can complete and have your plane fly as many slots as possible on every route
Avoid leasing aircraft, it costs money and lowers the value of your airline.