Ctrl+H show's and hide's extra's which marquee selections are considered. So if you have something selected and you want to make some adjustments, but the marching ants are obstructing your view, hit ctrl+h to hide them while maintaining that selection.
in CS and CS2, although this probably works on all version of PS: In the layers window Ctrl-Click a layer, then Ctrl-Click another layer *ABOVE* the first chosen layer then hit Ctrl-E (Merge). It will merge the bottom selected layer ON TOP of the top selected layer as if the bottom WAS on top already.