IMO, it is always an important factor to look for an airport with high domestic demands if you started late. Get an airport in a country having at least 50 airports, start with props on all the short distance domestic routes. This really help a lot because usually ~50 pax route is too insignificant for other players who started early and not many people fly them. Normally this will be flying to small airport and the slot fees is very very low compared to big airport, this save a lot for starting. The next is short distance but high demand (maybe >200 pax) route. Even if the route is flown, they might have establish a fleet of regional jet or jets like B737 which you can easily win the market by frequency if you are flying props. Assume that you are flying Fokker Fk.27, you can fly them up to 1000nm routes and you don't need to add DC9 or B737 too early, saving fleet commonality. Once you have fully filled all possible route with props, then only you get your 2nd type of aircraft. By that time, you'll have establish a strong base.
I started at Jakarta Kemayoran a class 3 airport having 85% domestic and 15% international short haul demand. Yet to reach 1 and half year, I fly 23 Fk. 27, 7 B727, 4 B733. All the B733 are new and I still have some in order. Having a fleet of 34 but 3 type is not very economical, but I got no choice because I have fully filled up all the possible routes. But yet I'm doing quiet well with $1m profit weekly.
Before started at Jakarta, I based at Guangzhou Baiyun and in 4 months time my fleet grow to 50 Fk. 27 + 7 B707. But I was restricted due to slots and I was too young to open a new base out there. So I end it up and start somewhere else.