Level 255 Pokemon
Chance of screwing up your game: Very likely - use with caution!

The problem with this cheat is that the level 255 Pokemon you get is random, and everybody's is different most of the time.

This cheat requires using the Item Duplication and Missingno cheat, so it might have the same effects as those two cheats.

Okay, first, you must have a rare candy.  If you've already used them up, there is a secret to get another one.  Go to Celadon City and there is a large house at the top of the city.   Go in there, then through the backdoor.  You should be in the backyard.   Now, walk back and forth across the whole backyard, and you should find a rare candy.  If you've already done this trick and don't have any left... sorry, you're out of luck.

Now that you have the rare candy, go to your item list and place it as the sixth item in the list.   You can do this by pressing select and pressing it again in the sixth place.   Next, you go to Viridian City.  At the top of the city, as you are leaving, there is an old man walking by.  Talk to him and tell him "no" that you are not in a hurry.  He'll show you how to catch Pokemon by catching a Weedle.  Now that you've done that, go to Cinnabar Islands.  (I know, there's too many places to go).  On the east coast of the island, swim there and make sure you are at the very coast where you are swimming on half-land, half-water.  Constantly swim up and down that area, and you will run into a Missingno.  Knock him out or run away.  Now, you will notice that your rare candies have been duplicated to 100-something.  Then, you keep swimming up and down and up and down, and you will eventually run into a Pokemon that is over level 100.  Most of the time, you will run into lots of Missingno's, but just run away and keep swimming.  Okay, now, catch the +100 Pokemon.  Then you can use those rare candies to raise the level to 255!  If you go past 255, the Pokemon will go back to Level 0.  You don't necessarily have to raise it to level 255, you can use rare candies to any level you want to up to 255.

If any +100 Pokemon gain experience, the level will drop down to 100.  So this means that your +100 Pokemon can be used only when fighting other human trainers, because you don't gain any experience from fighting them.  Also, if you retreat the Pokemon, you have a (small) chance of freezing the game up.  But don't worry, nothing happens to your game permanently.

Duplicate Pokemon
Chance of screwing up your game: Very likely - use with caution!

Set up a trade with the Pokemon that you want to be duplicated and any other Pokémon that you don't want, becuase it will be erased. For example, if I wanted to duplicate my Mewtwo, I would set up a trade with a friend and trade Mewtwo for anything else, preferably something they don't want.   Near the end of the trade, the screen shows "Waiting..." then "Trade Complete!" When it says "Waiting," turn your Gameboy (the one with the Pokemon you want to duplicate) off when the "Waiting" screen disappears, do it quickly.  Then the other Gameboy will say "Trade Complete," this is when you turn that one off.  Now you will both have the Pokemon.

MissingNo.
Chance of screwing up your game: Very Likely, known to restart game or mess up saved game.

Go somewhere where you trade Pokemon with a computer trainer.  A good place is Cinnabar Island Lab... there are lots of people to trade with.  Trade with anyone, then go outside and swim on the left shore of the island, where you are swimming on half-land, half-water.  Keep on swimming up and down, and you will run into MissingNo.

'M -- The Other Missingno
Chance of screwing up your game: Very Likely, known to restart game or mess up saved game.

Go to Viridian City and go up until you run into an old man.  He will ask you if you are in a hurry.  Say "no."  Then, he'll show you how to catch Pokemon by catching a Weedle.   After that's over with, fy to Cinnabar Island, and swim up and down the right coast, just like in the first Missingno cheat.  You should run into him.   Sometimes he's at level 0, and sometimes, you might even run into a Pokemon over level 100.

Item Duplication
This cheat will give you a little over 100x of any item you want.  This trick requires you to perform either Missingno cheat.

First, you place the item you want to duplicate and switch it with the 6th item on the list (by using the Select button).  Then you do the Missingno cheat.  I'd prefer to use the 'M cheat becuase you don't have to trade everytime, you just have to watch him catch Pokemon.   Anyways, when you run into Missingno or 'M, run away or knock it out.  Once the battle is over, the 6th item in your list has a weird symbol with a number in front of it.  This happens just because the item is over 100, once you use it up to less than 100, it will return to normal.

PokeBall Acting As Master Ball
Using any Poké Ball, press up and B at the same time as soon as you see the ball flying toward the Pokémon. Let go of up and B when the Pokémon gets caught in the ball.  You should be able to catch it without letting it escape.

Some people say to keep on holding up and B, and some say to hold up and B when you see the Pokemon get caught in the ball.  There are lots of different things said about it.

Easy Ditto Catch
This isn't really a cheat, it's more of a handy tip for people trying to catch Ditto.

Before you run into Ditto, put Magikarp at the top of your Pokemon list.  That way, when you run into Ditto, he'll Transform into Magikarp, and he'll be easy to fight or catch.

Cheap Trick - Standing On Top Of Trees
Cut down a bush stand on the spot where the bush is. Save and restart the game and you'll find yourself on top and stuck right there in the tree. It's funny, and it makes no sense whatsoever. So what's the purpose? It's cool!

Better Chance Catching Safari Zone Pokemon
When you throw a Safari Ball, the instant you see the ball materialize, start rotating the pad from the left in a counter-clockwise motion at a semi-slow speed. I have been able to catch the most difficult Pokemon in the Safari Zone multiple times. It's not 100% effective, but it is in the 80% area.