My preferred method of perspective adjustment is using the lens correction filter. However, if you use the transform tools, this method helps visualize the vertical correction that is needed after straightening vertical lines.
One circle (white) is stroked near the image center of a new layer. A duplicate circle (the same size and location) is stroked on a second new layer. A different color can be used, but not required.
Then target both the image layer and the first circle layer to perform the transformation.
If the top two corners are moved out approximately the same distance that the bottom is moved in, the side handles will fall in the original position on the edges of the image. This will maintain a "round" circle and good image proportions. If the circle doesn't appear round by uneven corner movement, the top and bottom center handles can be moved as required to reshape the circle. The Red circle just shows original reference point.
After the transformation is complete, delete the two circle layers.

forum discussion: http://forums.dpreview.com/forums/read.asp?forum=1006&message=27373376
original image by David Cole: