So...not sure if you consider this cheating, but here's my method:

I use a projector to project the image onto my canvas and then use a pencil to trace the necessary shape:

1. Take a good photo and bring the photograph into Photoshop. If there is stuff in the background you can just ignore it when you do the tracing on the canvas. Resize/crop/center the photo so that the image matches the dimensions of the canvas. (Create new PS file in the size of the canvas at 150dpi and then just paste the photo into and and resize to fit.) Save the image.
   

You now have 2 choices: You can project the photo directly on the canvas and trace and shade to match the photo. This has mixed results. I find that the shading is always off when I use this method. Or you can edit the photo to separate shading levels:

1. Use the Cutout filter (Filter>Filter Gallery>Artistic>Cutout) This will convert the image to a limited color pallet. The number of colors is limited by the levels. So 4 levels = 4 colors. So for a shaded area that goes from dark to light, it will separate the area n to 4 distinct levels of shading. So you will now exactly how much to shade in which particular area. You can increase the number of levels but as you do so, it becomes more complex and less defined. 4 to 6 levels is optimal in most cases.
2. You can then project this image directly onto the canvas and start shading with the pencil now having more control over how to properly shade.
   

The shaded image you have now drawn onto the canvas should look pretty good. Now you just need to paint on-top of it. Since the pencil graphite can sometimes blend into the paint and discolor it, you might need to spray a light fixative on top or a clear coat. Depends on your painting style and thickness of paint you apply.

With this method, you should generally work the levels from dark to light. To get rid of the "contour map effect" just blend the boundaries of the shaded areas well. So apply paint to the dark areas first. Then paint the next area down and blend, repeat until all levels are completed. You can then finish the painting by comparing to the original photograph and adding touches of highlights as necessary (but I don't recommend adding additional shading.) Your end result should be pretty good.

A few additional tips:

A. A cheap projector typically works fine. You can use a nice digital projector or a cheap overhead projector and print on transparency paper.
B. Try to project onto the canvas dead-on. Meaning the projector is at level with the canvas. The reason is that the canvas has some "give" and the angle of the projection gets messed up. Meaning that when you apply the pencil, the image you draw will be a centimeter or 2 from where it belongs. Small digital projectors typically shoot upwards at an angle and the distortion is fixed using the keystone setting. To get it dead-on you might need to tilt the projector by stacking a book or two under the back of it.
C. If you know Photoshop, you can edit-in the background of your choice. An interesting portrait has an interesting background. Compare to the Mona Lisa's background.