Make a new layer, and brush whatever you'd like the shape of the clipping mask to be in black or white. Hide that layer. Go to Edit -> Copy Visible, then go to Edit -> Paste. Anchor that layer by creating a new one, and right click on the layer with your black/white brushing is. Choose "Alpha to Selection," then go to the Copy Visible'd layer. Invert the selection, and delete. Then just move around the Copy Visible'd layer. If that doesn't work, follow Toshi's links.
