This is ridiculous. There is no sure-fire, 100%, concrete way to ensure that something like the Green Lantern titles will turn into a constant top selling series indefinitely.
Putting a top tier creative team on the series and moving it in a new direction, a notion rejected in the OP, hasn't served characters like Batman too badly in the past and is the nearest that you'll get to a best case scenario. The much lauded reboot idea (that's cropped up on many threads now) is no more of a problem solver than any of the other 'band-aid' solutions that have been disparaged on here. As we all know, comic book fans in general just can't enough reboots
For a start, it would help the franchise if DC actually got around to focusing on it more. It's not been a priority for them for a couple of years now, certainly during Rebirth. The various GL titles and characters have had little interaction with and even less of an impact on the ongoing 'Rebirth/Doomsday Clock' narrative and are a bit of an afterthought because of that. This is frustrating for GL fans but won't be the case forever.