I used to make my sketches with a CBic Matic pencil (0.7mm), but they made me discover the Staedtler Mars Lumograph Graphite Pencil 6B (the fact of giving the technical names of pencils gives the impression to work for NASA…).
I must confess that a good pencil, heightened with a sharpener, changes the game. The fact of having a very oily pencil makes it possible to let your imagination slip on the paper, and therefore to see shapes appear, which you would not have seen with a pencil too dry.