I moved away from this sim for more than a year, but decided to come back to it for a little management fun. Well, what was said earlier has pretty much been the case with me. I came in at the middle to late in MT#5, and no matter what I tried I lost money. I tried all the tips and tricks, read the FAQs, read up on the forums, but no luck. I tried small planes, large planes, under- or un-utilized markets, non-competing routes, single routes, multiple routes, high-cost/high access marketing, low-cost/minor access marketing... It really seems that, unless you get into a game right near the beginning, there's no way to make enough money to grow.
And after a while, even as route image gets to the point where you're getting some reasonable returns, and as you limp along breaking even, one of the established airlines comes in putting 3x capacity on your routes (check out KMCI-KMSP for example) which cuts your load factors by 1/3 to 1/2 (even with 100 route image), leaving you with a smouldering heap of airline that's $5m in the red.
Five attempts, five BK's, and all the help and tips couldn't make a late-in-the-game entrant viable, let alone profitable.