What is the most professional way to go about creating an accurate portrait from

Question:

What is the most professional way to go about creating an accurate portrait from a photo?

The ways I'm already aware of are: the grid method, tracing using a light box, using an opaque projector, drawing over a very light computer aided printout, and the old fashioned drawing from sight ("trial and error").
Please tell me what methods (whether I listed them or not) are used by a professional portrait artist. Which methods are good for a artist wanting to start out doing cheap portraits, or any other advice you have. For example, if you recommend tracing, what is the best tool to use (ie light box? what model?)?
Also, if this helps, I'm wanting to use prismacolor colored pencils as a medium.
Thanks!

Best Answer - Chosen by Asker:

Tracing really is the best method. The best portrait painters used camera obscuras. You could use that, I think they run about 20$ at an art store. Or an overhead projector also.