It depends on lighting, scale, etc., but the way I usually do it is to lasso around what you want to move, copy it, then paste it in the target image. Then when you have your object selected clean up the edges with an eraser. Usually works ok.
After I made the intial post I was reminded of a photo I put together a couple of days ago.. I needed a photo of a bong rip but couldn't decide which tube to use. So I used both of them. Kind of a similar situation.