If the game were a movie, how much "screen time" (that is, being the primary focus of the camera, the principal character the scene is built around) is a given Player Character getting? Though in movies there's usually a hierarchy where lead actors are going to get more screen time than supporting ones, in tabletop games, where every player expects an oppurtunity to contribute, its generally a good idea to give everyone an equal chance to shine (Some may not want to shine, and that's ok, but they should get the opportunity).
Primetime Adventures has an explicit "Star Power" mechanic so that the ensemble cast of your story each officially gets their opportunity to get a lot of screen time and shine at some point during the campaign.
Universalis gives every player coins which they spend on narrative power; although good players can game the system to get themselves more, the players who don't use their narrative power at first usually will find themselves with narrative power to burn later.
In games with a GM, the GM should try to spread the attention around evenly: have bangs for every player. In games with artha, hero points, or XP-for-hitting-keys, you can represent the points with chips or tokens, and the player with the smallest pile of tokens is the one who is getting passed by. Give them a bang.