well try this to..
this will give realy impresive results if the object is surounded by light colors
1. duplicate the layer
2. convert it to gray sclay
3. play with levels so that the object becomes darker and surrounding get whiter
4.go to chanels and make any of the chanel as selection.
5. go to original image apply mask(make sure the selection is still loded)
voola
another method
read this
http://www.phong.com/tutorials/mask.tree/
keep experimenting