Duplicate Items
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" (Missingno.) Once you see him, kill him or run away. DO NOT CATCH HIM! After that, you should have multiple copies of your sixth item!
Extra Damage
While fighting, select your desired attack. When it says you are going to use that attack (eg. "Venusaur used Razor Leaf"), hold down the A button until your opponent's life stops draining. If your timing was right you'll do more damage than usual.
Get MissingNo.
Warning! Catch missingno in the red version may delete your old game and prevent you from saving any other games.
To get MissingNo (a glitch) go to Cinnabar Island and go into the Pokémon Research Labratory and trade with anyone in the first room. Then leave the Lab and go to the EAST of Cinnabar Island and surf up and down on the part where the land meets the water. Eventually the battle screen will come up and the screen goes black for about 5 seconds.
Once you find him I suggest you use a Level 30 to 40 Pokemon. This is because Missing No., even at Lv. 80, has a VERY low defense rating. So get him down pretty low. Even at low he is still extremely difficult to catch. So next get a Pokemon that knows a sleep move and put him to sleep. Follow this up with a couple Ultra Balls and you should have him.
Level 100+ Pokemon
Follow these instructions to catch pokemon with really high levels.
Talk to the old man in Viridian City who shows you how to catch pokemon.
Fly directly to Cinnabar Island.
Put your strongest pokemon on your team.
You might want to save now.
Surf on the shore where the land touches water.
Keep Surfing until you run into one of these strong pokemon.
These pokemon are well above level 100. Unfortunately, they will revert back to Level 100 if they gain any experience points. Rare Candy is the only thing that raises their level. They can be used in a two-player battle without losing their level, so have fun liquidating your friends!
Hints/Tips
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).
Celadon City Slot Tip
All of the slot machines have different odds, which change every time. Play each machine 4 times, and if it hits 2 or more, then stick with that one, because it's probably hot.
If the slow machine forces you really close to a 777 or Bar Bar Bar it will probably give you one or the other soon.
