I use pencils to do thumbnails, but then I go full digital - using a combination of Manga Studio 4EX and Manga Studio 5 (i.e: Clip Studio).
Well, uh, I have partial face-blindness, so I kind of don't when it comes to faces - I do my best to! I just can't always manage, because my brain works weird.
But in general, when it comes to consistent details in clothing and tattoos and scars and whatnot - I usually spend a lot of time scribbling and drawing in a sketchbook until I start getting the feel for how they should look, and then I draw a reference sheet for myself, where I draw them, in detail, from the front, back and sides, so I know what they look like from different angles. Then I keep that reference sheet on hand while I draw.