Go to Floaroma Town
Above the Pokemon Health Center there will be a blonde haired woman. Talk to her and she will give you a G Flower. It will be in your Key Pocket and the G Flower is only allowed to be used during the morning although it adds a good touch to Shaymin.
What it does: Turns Shaymin into Sky form. Shaymin will be Grass/Flying and it will look different and epic.