Few notes as I play around:
Finances -> Try not to have high-value aircraft leases in the first few days of the month. The game tends to suck out all your gate fees, maintenance fees, and others at the 1st & 13/14th of each month. Which means you need to stockpile money after the 20th. But if there's not enough money, any leases / bonds due at the front end of the month will not clear and you risk default.
Days of months -> It seems the game operates off 24 days a month, NOT 30.
On Time Performance -> I've found if I leave about 20 hours free on each airframe, that tends to help keep On Time in line.
Inflight Service -> I use F/C in smaller cabins, but have a different service for those where they get free food (level2/3) but can pay for level 5.
IFE -> Same as above, I can differentiate between awesome free stuff for premium cabin vs pay for it in back. I try to keep IFE settings at 3 and ads at 1 hour.
Gates -> some are SUPER duper expensive, and that money comes out at random it appears. I try to maximize my slots at all airports and connect dots if the game scenario allows it. This is especially important at very large/expensive airports like LHR, PVG, ORD, etc. building terminals help reduce this cost, but once you make it a HUB, those costs go up again (still cheaper than if you do just gates).