High speed belt sander. Hand sanding has always just brought up the nap for me. The belt sander has done well to tear off the fleshy nap, just try to be even and know that it'll reduce thickness, not a lot necessarily, but some. Without a belt sander I got nothing but headaches.
I'm very happy with the buffed leather I have right now, but there are certainly differences across the piece. Some is stiffer, some softer, some very smooth on the back, some not as much. On pouches (and wrist cuffs) I use the smooth stuff. If they are gonna be lined with pigskin I use the nappier stuff which is good for the contact cement that I'll use.
It's like selecting the piece for stamping or tooling and dyeing. If you are doing that anyway, may as well use a piece with stretch marks.
Edit- I agree with James btw. I have often slicked, but it can only do so much. If it's nappy to start with it'll probably lift the nap at the bend in the loop which can make it look icky (to me), but slicking a nice smooth nap can make it look almost as good as the skin.