EDIT: I got distracted four times while trying to write this, hopefully it makes sense, but ask for more clarification if you like.
Nickel and diming is good in the first few days of the game when just about every hour counts and monthly expenses need to be kept under control, but six months in you don't need to be choosing planes based on a few thousand euro profit difference. You can spend all that time pouring through the aircraft list and punching numbers on your little calculator, but as soon as some wanker puts a larger plane on the route, you're stuck with your pants around your ankles and your farts aren't making noise any more.
When it comes to leasing, I ONLY lease in the first six to nine months in order to get the ball rolling. Early on though, lease rates are extremely high due to supply and demand issues so in order to keep it all in check, I only lease planes with rates that I can pay off in four to five game days, keeping the profit on the plane over 75%.
I'm the king of early lease turn ins too. Nine times out of ten, I'll buy a plane, then try to lease the same plane off the open market for a day or so to retain my gates and make a little profit.
Think not only about how you're going to make the most money you can, but think about how you can siphon money away from your competitors too.