At this point in the game, Espeon, the only ways to grind for EXP at the point you're in that I can think of is:
- Battling any trainers you may have missed or more likely skipped.
- Plain old grinding on wild Pokemon that appear near Fuschia, as those are of the highest level that you can find at the moment until you beat Sabrina.
Another tip: Use Psychic, Ground, and even Fire, Rock, and Flying-types to get the best out of defeating Koga, as all of his Pokemon are of the Poison-type, including some Bugs.
Source: Experience and knowledge