Using Selection Tools in Photoshop

When I select part of a photograph using, say a oblong selection tool, I get the marching ants. Is there a way that I can get the marching ants but also black out the area not selected. I find it difficult to visualise the finished object until I crop, then often need to go back and reposition the selection box. I would like to be able to move the selected area around, with the background blacked out to get my composition correct, then crop the part I want. I hope you understand what I mean.