I like my cheese filling to be fairly light and fluffy so I don't use any of the stuffing techniques mentioned above. I simply blend my cheeses, etc., together with a fork using a light touch and and stuff the manicotti, pasta shells, lasagna, or whatever with the same fork. I generally prefer stuffed shells to manicotti because they're easier to stuff and I think they make a more attractive presentation.
My "standard" cheese filling is made with equal parts of mozzarella, ricotta and parmesan cheeses, chopped parsley, salt and pepper, a touch of freshly ground nutmeg, and a raw egg.
"Iím going to break one of the rules of the trade here. Iím going to tell you some of the secrets of improvisation. Just remember ó itís always a good idea to follow the directions exactly the first time you try a recipe. But from then on, youíre on your own." - James Beard