I totally support your quest, OP, and hopefully somebody on here will have the trick you're looking for. I sympathize because my sack is almost always tight, and I'm also kinda jealous of guys with awesome low-hangers.
But...my advice is that you should try to reconcile with your tight-ass balls and learn to love them as they are. Because I'm pretty sure (from experience) it's often a permanent, innate feature which you can't really change. My balls will never swing low, and I've just had to accept that fact.
I figure you're like me because I find it amusing when all these posters (god love 'em) talk about hot showers and thermal underwear, etc. You guys are lucky, but that's definitely not universal. Like OP, my nutsack is totally unimpressed by hot tubs, saunas, 3 layers of snowboarding gear in front of a roaring fire, whatever.
Of course, you can try all the body mod type stuff—like weights and rings and stretchers, etc.—but I have a feeling that's not what OP has in mind. Actually, I suspect you dudes who play around with ball stretching stuff already had pretty rad sacks to begin with!
Anyway, I've got a massive throbber that shoots a canon-load, so this isn't a self-pity post. Always look on the bright side, huh!?