Cut Down Grassy Areas
Once you get the HM "cut", your pokemon can cut down grassy areas. Use "cut" and it will cut a two by two area. Wild Pokemon will not attack you in the area you cut.

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!

Fight Land Creatures in Water
To fight creatures that normally live on land in water, go to the water around Cinnabar Island or Seafoam Islands (The only place you can be randomly attacked while swimming.) Swim along the shore of any island and you will fight land creatures. (You have to swim on a place where the square is half land and half water.)

Instead of fighting a set group of monsters in the water, you actually fight monsters from the last place you were. So to get safari zone monsters, you need to go to the section of the safari zone with that monster, let your time run out, then go to the coast. Using this trick I have caught Kangaskhan, Scyther, and Tauros!

Get MissingNo.
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.