How I cook any tender greens (spinach, swiss chard, beet greens):
Stem greens and clean in a sink full of water. Mince 2 cloves garlic and heat in a large saute pan with 2 Tablespoons olive oil. When garlic starts to turn golden, add wet greens; cover and cook, stirring occasionally, until completely wilted but still bright green, 3 to 5 minutes.
The book The Perfect Recipe by Pam Anderson has some really good recipes for cooking greens, both the tender kind and the longer cooking ones like collards and kale. Most of the other recipes in the book are meaty types, so it's probably not something you'd want to buy, but if you can find it at the library it's a good way to get some ideas.