My $.02...
Well, I've actually used Don Hume's pouches to carry my ASP(it's actually a PPCT Phoenix baton, but it's the same size as a 21" ASP), and here's what I have to say on the subject:
1) The open-top pouch pictured in KPS' post fits VERY tightly, to start. This is due to new-leather tightness, wet-molding to the exact model of baton ordered, and a dime-sized nub of foam covered by leather on the inside of the pouch that provides added tension/friction. If you get one of these pouches, I suggest that you jam your ASP in as deeply as you humanly can(it WILL go all the way in, you'll just have to muscle it the first time), and leave the baton inside the pouch for at least a day, to allow the leather to stretch and the foam insert to compress. I also suggest ordering the smallest pouch(16" baton) available - it will expose more of the handle, allowing a better drawing grasp and reducing friction, especially if you decide to carry a 21" or 26" baton. The longer batons carry just fine and stay perfectly secure in the shorter pouch. The snap-on belt loop option makes the pouch very convenient to don/doff. The loop is usually sized for a 2.25" duty belt, but it hasn't caused me any significant problems with shifting - you may wish to try anchoring it between, say, a pager and a belt loop on your trousers, or with keepers.
2) If I were carrying an ASP as my primary self-defense weapon(i.e. NO GUNS for some reason), I would use Don Hume's "Sur-Release" baton pouch - the baton is retained by a leather cup and a snap-secured leather band around the handle, which allows for VERY secure retention AND a fast, positive draw. I find that this rig carries and draws best just behind my strong-side hip, like my pistol - and there's the rub: the defensive handgun I usually carry holstered there interferes with the baton, or vice-versa! So, I only carry the baton when I can't/don't want to carry my primary handgun.(BTW, the 16" Sur-Release works REAL good for carrying a Sure-Fire Millenium M3 or any other 9V tactical flashlight.)
3) Back-pocket carry also works OK, depending on how deep said pocket is on the pants I happen to be wearing - the deeper, the better; the shallower, the less secure. On a couple of occasions, I have had to recover the bloody thing from the seat of my car or a bench in the mall - that's why I usually prefer pouch carry.
4) Currently, I'm using one of Don Hume's newest products - a combination magazine/baton pouch(Model D425A-M/B). So far, it's working VERY well - for the concealment-minded(like me), it carries the baton high and tight to your side, and absolutely minimizes printing when you bend over. The baton draws pretty smoothly, unless you have a short waist - it also helps to order the pouch for a 16" baton, as I mentioned earlier. If nothing else, this pouch enables me to carry my baton and my pistol at the same time, without unduly compromising either weapon.
HTH, Kor