If you’re doing client work, the very best idea would be to invest in the best tool for retop in Blender, Retopoflow:
https://cgcookie.com/courses/retopology-with-retopoflow-3
I think the most comprehensive retopology session would be in the Sci-Fi Helmet course, but it’s also an older course. Full of timeless knowledge though:
https://cgcookie.com/courses/modeling-a-sci-fi-helmet-in-blender
