Hey Paulo, agh don't know how comfortable I'd say I am but I did do a lot of figure drawing in college, and over the years. What did help the most was figure sculpture. I did a life size figure and several smaller figures, understanding the body in 3D will help leaps and bounds in constructing a body from the imagination. The rest is all just remembering proportions and perspective as well as what sort of detail is ok to leave out. Essentially, think big shapes first and think in 3D, it'll be a big help.
Here're some more sketches from the past couple of days.