Note: These cheats are for Ruby, Sapphire, Emerald, FireRed and LeafGreen. Not Diamond, Pearl, Red, Blue, Green, Yellow, Gold, Sliver or Crystal!
There's a special way to do this cheat in FR/LG. I find it better, but statistically, all of them have exactly the same effectiveness. You can also use the R/S/E cheat in FR/LG. Follow the steps:
1 Choose 3 pokemon that you want to get to level 100 (you don’t have to put 3 in if you don’t want to. It's just faster this way).
2 Give each pokemon to one of the daycares (one just south of Cerulean city and two in the daycare on four island).
3 Go to Five Island.
4 Enter the Rocket Warehouse.
5 Once inside head to the set of 3 arrows pointing left, step onto it.
6 Take another step left to go onto another set of arrows.
7 Head right until you come to a set of 3 arrows pointing down, step onto it.
8 Then take a step right to go onto another set of arrows.
9 Once stopped, get a small weight and press it onto the up key on the pad** (make sure it is stable).
10 Your character will start moving up, only to be knocked backwards by the arrows, this accumulates steps.
11 Put your gameboy on charge and leave.
12 Come back later and check your pokemon in the daycare(s), they should have levelled up considerably.
Ruby/Sapphire
As Ruby and Sapphire lack a Team Rocket Base, we have to improvise. Follow the steps:
1 Choose 2 pokemon that you want to get to level 100 (you don’t have to put 2 in if you don’t want to. It's just faster this way).
2 Give the pokemon to the daycare (Just west of Mauville City).
3 Go to a place that has either a Slippy Mud Slide (The things that you use a Mach Bike to ride over) or a Waterfall base.
4 Stand and face the mud/waterfall. Get a small weight and press it onto the up key on the pad** (make sure it is stable).
5 Your character will start moving up, only to be knocked backwards by the arrows, this accumulates steps.
6 Put your gameboy on charge and leave.
7 Come back later and check your pokemon in the daycare, they should have levelled up considerably.
Extra things
Make sure your party is free from Eggs. If an egg hatches, your character will stop walking. Don't waste your time like this!
Amazingly, this cheat doubles as a Happiness Cheat. Put any Pokemon that levels up through happiness in your team, then commence the cheat. It's happiness will go up, then all you have to do is level up once and it will evolve. (Great for getting Umbreon and Espeon on R/S!)
This cheat is not recommended for Pokemon Emerald. As you would know, people ring you as you walk through the region. If a person calls you as you are doing this, your character will stop. Don't waste your time like this, either!
