a little detail :
How many fleet types do you have?
Not the biggest trouble early in the game, but can indeed be a killer later
Are you getting old aircraft that cost a lot in maintenance?
Be especially careful about the D-Checks. If you loan old steel that has D-Check within 3.4 years, don't loan more than 3 years. And don't extend.
Do you try to maximise individual plane schedules? (avoid arrivals and departures between 23:00 hrs to 5:00 hrs)
ideally, you fly East at 23h00 and arrive after 05H00. depends on your fleet, but it really can help to use your aircraft more efficiently
Do you overspend on marketing for the size of your operations?
nearly killed me once. Stay under 10% for CI, and keep RI marketing for lines with a lot of traffic.