Find a good location to take the photo and set the camera up on a tripod with a self-timer and on multi-shot. take a photo of yourself standing, sitting, or laying down on something that doesn't get in the way between you and the camera. I used recycling bins since they were right there.
After the first multi-shot photo is taken, get off whatever you are on and move yourself and it completely out of the frame and continue the multi-shot function long enough to get a clear shot of the background.
Open GIMP. (A computer program that can do similar stuff to photoshop, but it's free. Download it at http://www.gimp.org/downloads/ ).
file>open: a photo of the background.
--file> open as layers: the best photo of you. Make sure it is the top layer in the layers box on the right side.
--layer> transparency> add alpha channel.
Click on the layer with the picture of yourself in the layers window. Use the free select tool (third in from the top right in the toolbox) to select the general area around the thing you're sitting on, making sure not to include you and your clothes. Zooming in is a good idea.
press ctrl+x to delete the object and show through to the background.
Voila! A picture of you floating! Save it of course, and admire your work. I usually like to add some filters or effects in a photo editor to give it a magical feel. I use pixlr o-matic. Done!