Pokémon FireRed and LeafGreen/Route 14-15

After passing through Routes 12 and 13, you'll end up in Routes 14 and 15.

Professor Oak's Aide
Inside the Guard Post building on Route 15, east of Fuchsia City, you will find one of Oak's Aides upstairs. Talk to him, and if you have complete data on 50 types of Pokémon, he will reward you with an EXP. Share. Giving this item to a Pokémon allows it to receive half the experience from every battle without bringing it out. For example, if your lead Pokemon gets 200 EXP, the Pokemon holding the EXP share will get 100 EXP. If the Pokemon holding the EXP share participates in battle, it will get double EXP.

Share and Share Alike
Just before returning to Fuchsia City, go to the lookout station above where the guard stands and talk with the Professor's Aide. If you have caught more than 50 kinds of Pokémon, he'll give you the EXP Share! Unlike the EXP All in Generation I, this is a hold item, and only the held Pokémon shares the experience and Effort Value Points with the Pokémon that battled. This is much less time-consuming and aggravating than having to go through six EXP screens, though.