Funny you should ask Guaruj, I just read this yesterday from Justin in Aug in this tread:
Keurig cups - I really love the convenience of my Keurig, but those cups of coffee aren't cheap. I supplement store-bought K-cups with reusuable cups and paper filters purchased on Amazon. Use a fine-ground coffee (I like Cafe Bustelo) and pack the cup tight. There are also cups with built-in screens, but I have high cholesterol and need the paper filter to remove oils.