[Andy]

You can switch into vertex select mode and use that to select say, the top or bottom vertices of the box from the front or side views. This means that when you use a function in the surface menu, the actual surfaces that have all their vertices selected are the ones that will be worked on.

You can switch from vertex to surface mode and any surfaces that have all their vertices selected will become selected surfaces.

The easiest way to select a partiular surface (in Surface select mode) is probably to hold down the control key and click on the surface in the 3d window.

Andy