I used to have this problem (not so much with thongs but with underwear in general), but when I bought new pants I started to do things like squat and sit down when I tried them on in the dressing room, to see how well they fit. If I didn't like the way they fit or moved while I was sitting then I just wouldn't get them. This really helped cut down on this problem. Some people suggest wearing a belt but that's never really helped me, personally.
You could also: -wear shirts that are a bit longer. -stop wearing thongs. -wear jeans with a higher waist.
you don't see it as much as you used to , I remember back in high school you always be seeing girls pants and they'd always be showing off underware when they bent over , now you don't see this as much I find. myself I don't think I'm showing off any butt cleveage
I have sort of the same problem, because I have a small upper body and a larger lower body, so it's hard to find jeans that are loose enough around my thighs/butt and tight enough around my waist. You can try high rise jeans. I've found that with some pants though, a belt is the only thing that fixes the problem.
I also have this problem! I've pretty much stopped wearing thongs because of it. I chalked it up to an odd body shape. Like you, I'm fit and active, no extra weight around the waist but my pants never fit me right. It's like thighs, waist, or butt pick one. Usually my pants have a death grip on my thighs, more or less fit in the butt and then gape at the waist. I wear belts and it sometimes helps.