When modeling, it is always wise to begin with orthographic reference which is scanned and placed in the front/side/top views of your 3D package. Nothing new to most of you, I'm sure, but this little Photoshop tip tends to fill people with a tingly warm all over feeling.

When an image is initially scanned and imported into Photoshop, it will inevitably be crooked as we see in the above image. So, it needs to be rotated. Straightening it by guessing with the Rotate Canvas tool is time consuming, especially on large images.

The image above was straightened by using the 'line' tool. While drawing a line with this tool, look at the Info pallette. In the upper right you'll notice that it is displaying the Angle created between this line and a global axis. So if you use the Line tool over a line in your scan which you know should be perfectly vertical or horizontal, you can determine the exact angle that the document needs to be rotated in order to straighten the image.

Another technique which works well with Photoshop 6+ is to use the 'Measure Tool' found under the eyedropper tool. Once tracing over one of the grid lines, simply jump to Image/RotateCanvas/Arbitrary and the correct value will already be inserted.