CI ONLY goes up with Marketing spend.
All the on-time flights, all the new equipment, all the non-cancelled flights in the world won't make it go up one bit. They make it go down if they're (really) bad. But never up.
If you want a higher CI you're gonna have to spend money on it. And, yes, it's expensive. For me to get north of 90 requires making Marketing spend my single largest expense -- more than fuel, more than salaries, more than anything else.
Remember, your CI Marketing spend is a function of the number of routes you operate. So if you fly a lot of routes (i.e. a more regional operation) you may as well give up on ever getting a CI of 100, or probably even much more than 60-70. The easiest way to get to a CI of 100 with the most minimal expense possible is to fly the fewest number of densest route that you can -- so that you're moving the same pax on 4 routes as someone else is on 40 routes.