Randomly changes as you go through dungeon floors.
You need to give it the Griseous Orb to hold, and it will automatically change forme.
I hope this Answer is better then the answer the other guy did, but I believe one of the places where Giratina turns Origin Forme is Star Cave. I do know there is other places, but I can't remember what they are.
Get the flower and use it on Shaymin to change forme.
A Shaymin in Shaymin Village gives you a Gracidea. Use that while in a dungeon and he will change form.
Go to shaymin village and talk to the shaymin near the fence, near The kangaskhan rock
Deoxys' Forme randomly changes as you progress through a dungeon. The changes are purely cosmetic, except for the Speed Forme, which doubles his movement speed until you move to the next floor.
Talk to one of the Shaymin in the village. At the top-right, I believe. It'll give you a Gracidea, which changes Shaymin's forme when used in a dungeon.
no shaymin's sky forme is exclusive to platinum
Yes, you can change Shaymin while it is in Land Forme over to its Sky Forme in Pokémon White however you will need the Gracidea in order for it to change it forms. You can get the Gracidea from a character in the Lacunosa Town Pokémon Center.
shaymin needs to have a gracedia flower to change into sky forme but it will return to land form when it is dark or if it comes into contact with ice, im not sure if it can change formes on Pokemon pearl though
Shaymin is a type of Pokemon. The land forme is a forme he can switch to.
Landorus Therian Forme in Pokemon is a basic Pokemon. It has the ability to intimidate opponents in battle.