I have been playing this game for a while now and after playing it several times I figured out the best way for myself.You may lose interest in the game( if you are still playing this O.o) after reading this because of ever increasing income from the very first day.
So lets begin with first day steps ( I assume it's not saturday or sunday).
-->You start with any home airport. For me I had changed to berlin but it's not necessary.You go to the routes screen and buy any route you like that your Boeing 757 can cover ( Funny as I always considered the daily demand and other stats but they are not sound factors buying a route :) . Right now I have almost all the routes available in the game). I bought Berlin-Moscow on first day.
--> At 11:00 go to the museum and sell Boeing 737.
--> Go to the bank and issue new stocks.Take loan from the bank and you should have some 8 million cash by now.
-->Upgrade you 757 to maximum (not possible with 8 million cash but start from comfort features and then technical equipments.Spend around 6 million on it
-->Go to the advertising agency and improve your route's image to maximum (almost spend 2.8 million if you have left otherwise 1.4 million.Believe me you will get enough money from first day income to keep things running :) ).
--> Now the final step is magical.Come back to your office and open globe.Go to your routes and open one of it.Set your 2nd class ticket price to dumping and raise your first class ticket price to as high as possible.For Berlin-Moscow route ,the limit is around 8k-9k.Oh and don't forget to convert your all plane seats into first class. :).

You will be earning ~900k from each flight if your route is berlin to moscow.Infact I completed all the missions with this technique.The reason I had changed the home airport to berlin was because I could open london routes later in game so that I don't have unavailable gates problem.( You need 20% of the daily requirement (Berlin-London) be covered to open the routes of london)