Actually just wait until night without shaymin holding the flower thing
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It changes back to Land Form at Nighttime and It goes into Sky Form in the Daytime. If you don't want Sky Forme at all then start over without a Gracidea Flower.
Actually you can trade Shaymin and Giratina to Diamond/Pearl from Platinum but if you do - Shaymin will revert back to its Land Forme and Giratina will revert back to its Altered Forme.
Well, first of all, I have to mention the error that I just corrected in the question. SHAYMIN is a Legendary Pokemon. CHARMIN is a brand of toilet paper. Now that that's been cleared up, let me first explain that Shaymin does not evolve. It changes form like Deoxys and Arceus. You must have an EVENT Shaymin, and travel to Floaroma Town. Somewhere in Floaroma, with the Event Shaymin in your party, you will receive a Gracidea Flower after talking to someone. Exposing this to Shaymin will change its form to Sky Forme and back to Land Forme. Shaymin cannot be in Sky Forme if it is frozen.
Get the Gracieda from a Beauty in FLoroma Town near the house in the center of the town and she will give you Gracieda and then use it on Shaymin and Viola it works.P.S. this only works in Platinum
Shaymin turns back to its original form when the gracidea flower (held item) is removed from it. It can also revert back to land forme during the night, and also if it is frozen. (which probably won't happen very often because it is grass/flying in its sky forme, therefore having a 4x weakness to ice type attacks.)