I put my packy in the fly part of my boxer briefs. That way there's a layer of cloth between me and it (cuz previously when it was skin-to-skin it was too hot and sweaty, eww), and it has it's own little "pouch" of sorts to rest in. When I drop trou to go to the washroom, it stays in its little nest quite well. When I pull up I have to give it a little "readjustment" to make sure it's in the right place, but otherwise it stays in place all day. It's the best thing since sliced bread if you ask me. I sleep with it in the fly too, and even without pants overtop to "hold it in" the fly holds it just fine. Never once woken up with it fallen out, even partially. I may need to adjust some mornings, but it never really goes too far - not even "ends up sideways" kind of adjustments, just "needs to be a little higher up" or such.
Just my two cents, ymmv. 🙂