Just in case you have missed these courses:
https://cgcookie.com/course/introduction-to-retopology
https://cgcookie.com/course/mesh-modeling-fundamentals
https://cgcookie.com/course/mesh-modeling-bootcamp
And for hard-surface modeling:
https://cgcookie.com/course/introduction-to-hard-surface-modeling
From my own experience: practice makes perfect. The more you model the better you understand the form of an object. You literally start to see the mesh and combine the whole as a sum of many different pieces.
A little piece of advice: try to model different things (organic/mechanic) and don’t use some fancy add-ons just for now. I know they can save you a lot of time (specially the ones like Extra Objects or Bolt Factiry). But you have to practice on such things first.
