Pokemon Red/Blue Cheats for Game Boy

Easy Exp points

Fight Elete 4 with for example Rattata level 6 or lower. By doing this it will give the pokemon a ton of exp points.

99 of Any Item

To get 99 of any item, first put the item you want to multiply in the sixth item slot. Now use FLY and go to Viridian City. Talk to the old man almost all the way north of the town. When he asks if you are in a hurry, say no and watch him catch a weedle.

Right after that, fly to Fuchsia city. Make your way south and, when you you reach the water's edge, use SURF and make your way to Seafoam island.

When you get to seafoam island, swim up and down the beach until you run into a Pokemon called "M" (a glitch like Missingno.) Once you see him, kill him or run away. DO NOT CATCH HIM! After that, you should have 99 of your sixth item!

Boss Tactics

Here are your best bets for beating the Pokemon Gym Leaders:

Brock: Use water or grass Pokemon against this rock trainer.
Misty: Use grass pokemon or electric to defeat this gym leader.
Leiutenant Surge: use a rock or ground pokemon and he's toast.
Erika: Use fire pokemon or flight against this plant lover and shatter her dreams of fighting you again.
Sabrina: Use bug or ghost against this psychomaniac and she is a goner.
Koga: This poison pokemon lover will be deafeted easily by using psy pokemon.
Blaine: This fire trainer will use fire pokemon against you so you can use water or ice.
Final boss: Giovanni, the team rocket leader, is the final gym leader - use water, ice, plant and he's out of there (and talk to him again and he says something to you).
In the Indigo Plateu you find 5 pokemon masters. Use this pokemon lineup for a better chance of success:

Plant
Water, Plant, Flight, Ice
Psy
Ice or Dragon
Your choice
Good Luck!