The higher it is kept, the more amount of flights an airline may have pre-scheduled. If you notice, currently most of your airlines can schedule up to 15 flights. As their rating increases, so does that number - I believe the max is 50, when at 100%.
To keep it high, I believe, you need to have planes depart with all the passengers who cleared security on the aircraft (if there are passengers not cleared through security, I believe this will not affect the rating). If you have “Always Send Away” enabled, I believe this can degrade the rating, if it is forcing planes to leave before completing the boarding process - based on who is already through security (watching the flight process panel “G” helps).
I could be wrong on the “how to improve” the rating, but I always notice a severely delayed flight, with 100% boarding completed will actually increase the rating, whereas I have seen an “on-time” departure with less than the full boarding procedure completed, and it always degrades my ratings. One would think the time of delay would factor into this, and maybe it will eventually, but I am 99% sure to improve a rating is dependent on how many ground service complete 100%, regardless of the time the game allots them to do so.