Get the largest amount of money you can hold! First get 100+ of any item, then sell them to the owner of the Pokemon Mart. Instead of pressing the up button 100+ times. When at number 00 go down 1 and you'll get the highest number of that item you can sell.
Catch Safari zone pokemon in the water! First go to the Safari Zone. Then FLY to Cinnibar Island. When in the water you'll run into safari zone pokemon
Start your game out with level 100 pokemon! How to do it: Get your friend to trade a bunch of Magikarps for your pokemon on your team. Start a new game and catch a bunch of weak pokemon. Trade the weak pokemon for your old team. Now you have level 100 pokemon and your only in pallet town!