Get a lot of Meowth's probably 5 and one of them is the Pokemon you'll be training and train the Pokemon with Wild Pokemon or by Trainerbattle by using a VS SEEKER, and because of meowths pickup you'll get Rare Candies (but not all the time) It can give some useful Items as well (eg. Full Restore, Revive, Carbos, berries, etc. )... :)
Get a Psychic-type Pokemon for the battle with Agatha she could defeat you with her Gengar, also If you already battled the Trainers in Victory Road, try rematching the Trainers In Kindle Road, especially the one's near the entrance of Ember Spa, get useful TM's from slots and in the celadon dept. store... :)
When battling the Elite 4 be sure to bring a lot of Revives and Full Restores... :)
Also catch the 3 Legendary Birds and Items can be useful as well like Shellbell, Leftovers, Charcoal, etc.
Get all useful Items you missed in caves, forests, etc... :)
Also you could post your team and their stats here so we can help you more... :)