I'm not a pro, but I had same problem. So I started checking "my aircraft" through "maintainance view" to see how many days there are left for C/D checks etc.
If several aircrafts are getting checks at the same time, I usually divide it out and start checks beforehand manually.
It costs you the same, but atleast all your routes will not be in checks at the same time, so You will still be generating some revenue. And once you have done this, your next checks will automatically be balanced out.
Another thing I started doing is that if I have capacity for 3 flights from A to B, I divide the route out on 3 different aircrafts so even if one of them is in C check- the route is still operating, and normally with increased LF.