depends on the era.
First, you have a stategic choice to make : turboprops, or jets? Jets are costiler per seat, bu allow more travels per plane per day. I'm always going for jets, but many players do have success with turboprops.
Then, be sure not to over-extend. For example, ERJs are "too small" above 600NM, Ejets above 1200NM(unless national flights, but as I play in the UK, I did not experience this). So avoid big destinations that are "too small", or could be soon.
Finally, make sur to fill your schedules. I just landed at Warsaw, and I've got lines with a lot of opposition; but as my planes are very busy(4-5 flights a day), it's still profitable. And if the 6am slots are all taken, you can "cheat" by flying overnight. You'll lose one day(because of the Acheck), but still 6 days per week for one more flight is good to take. It's worth only if your base airport is full, and the target is not, though, but I'm using it a lot from London City. It makes more bang for the same amount of planes.
Ah, and the choice of base airport is crucial. Average-sized ones without opposition are ideal, places like Prestwick, Beauvais, Reus... London City is a special case, and I would not advice it to a beginner. I won't come back there either, it's fun, profitable when well-used, but it's limited. I cannot even use properly this huge demand to Cairo.