When you first play, I would suggest NOT upgrading airports or planes. Bucks are used to buy parts for new planes. As you gain levels, the planes you can develop are gradually better. Develop better planes based on carrying more passengers, or the same passengers with a plane that has more speed / range.
Upgrading airports seems to me to be best kept for poor airports (0.4 style airports), so that when you have a few planes there, there are still jobs available.
Another important thing to remember is, try to keep a fleet of planes with a good mix of passengers / cargo. As in, don't just have all your planes for passengers. Try and keep a level mix between ability to carry cargo AND passengers, being as airports have a seemingly equal mix of passengers and cargo.
For early on - consider this:
- 'Watching' a plane fly earns you the most stuff. Regularly, coins and bucks fly across the screen to 'earn' by clicking them. (best way to earn bucks).
- Airports range from class 1-3. When you reach the point to develop a class 2 aircraft, they can only fly to class 2+ airports (blue & red). Not ideal unless you have a LOT of airports.
- When filling up your plane, don't just pick the first ones available. See how much profit you'll earn, go back and then try a different set of options for the best profit.
- Whenever a combo is available (i.e. your plane holds 3 passengers, and you can take 3 passengers to 1 single place) take it. The 25% bonus, plus the single destination gives a lot more profit.
- Whenever a buck passenger / cargo is available - take it. Even if it means a loss. Bucks are worth far more than a loss of coins.