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 Surfing Pikachu
Did you know that you can get a Pikachu that knows the Water technique Surf? Here's how you can get it (outside of winning a Nintendo Pokemon contest):
Surfing Pikachu is hidden in Pok�mon Stadium for the Nintendo 64. To teach your Pikachu Surf (a technique it can normally not learn), you need to first unlock the R-2 (Round 2) mode. Then do the following:
Enter the R-2 Prime Cup's Master Ball division (you have to beat the first three divisions first).
Assemble a team that contains a Pikachu that does not yet know Surf. You HAVE to use a Pikachu from your Game Boy game. You cannot unlock Surfing Pikachu using a Rental Pok�mon.
Do not register your team! Pick it directly from your Game Boy cartridge.
You now have to beat the Prime Cup's Master Ball division using a three-Pok�mon team that always contains Pikachu. Pikachu has to be in every one of the eight battles.
You are not allowed to use any continues. If you do, you won't get Surfing Pikachu.
Once you have fulfilled all the above conditions, you are shown an award screen with Pikachu standing on a surfboard. From here on, Pikachu can use the Surf technique both in battle and during gameplay (Game Boy versions only). In Pok�mon Yellow, you can now also access the hidden Surfing Pikachu mini-game on Route 19.
There is also a GameShark code that lets you access Surfing Pikachu, but we advise against using it, since it can mess up your Pok�dex and save data. It's not worth the trouble.