These are the levels you should keep your pokemon at if you want to be on pace to have a good team for the elite four, before you fight each Gym Leader, make sure all of the Pokemon are at least the level shown.

Brock: Levels 13+
Misty: Levels 25+
Lt. Serge: Levels 30+
Erika: Levels 37+
Koga: Levels 43+
Sabrina: Levels 47+
Blaine: Levels 50+
Giovanni: Levels 55+
Elite Four: Levels 60+