Once you collected your sample, it's important to get it dried ASAP - this is to preserve the nutrients as is in the sample. If you leave the sample to wilt and dry naturally, it will actually keep "respiring" as it slowly dies (morbid!), using up sugars in particular, which will give you a falsely low result.
Please be slow and CAREFUL when microwaving - it gets very hot and can go very quickly at the end. I don't want to hear about anyone setting their microwaves on fire!