Well, you can always use Feather option for the selection. For instance, use 1-2 pixels. That turns the 1-2 pixels at the edge of the selection progressively more transparent towards the edge, with the full transparency starting at the edge. Makes blending more natural. But, what I do is basically the opposite from what ricky does. :-) I render the mask, open it in Photoshop, select all, paste it onto a new layer, select black, invert the selection (Ctrl+Shift+I), and paste the object into the selection (Shift+Ctrl+V). Then I delete the mask layer, cause the pasting of the object into the selection automatically creates a mask and the mask layer makes it impossible to see what's under it. Also, you can then Apply the mask (right-click on the layer in Layer pallette, on the pic, not on the mask symbol) so you've got only the selected bit in layer. Perfect Skin was created that way. One, two... three different masks.

-- erlik