I agree with the above comment(s) about using a restrictor like a condom. If you're going for girth, be very patient and lower your suction (down to 5" etc.) so that your gains are more even throughout the penis.
If you want to use the condom technique, cut off the tip and slip about 1-2" onto your foreskin, fold id back on itself 2x and then cut off the remainder. It seems wasteful but it works (after some practice).
Really the best practices are (a) time, (b) patience, and (c) the correct cylinder. If you're going for length, you need a thinner cylinder and a LOT of patience. If the cylinder volume is too great for your current penis displacement, you'll see a lot of edema and not in regular patterns, often random, but especially in the foreskin since it's so stretchy.
(sorry for the long reply!)