Most Helpful Girl
When you buy pants, do you try them on in stores?
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.