Most Helpful Girl
I used to have that problem a lot. The issue doesn't come up because of poor-fitting thongs, but more because of your pants. When you're wearing skinnies or jeans, make sure to wear a belt so that when you lean over or sit down they don't ride down too much. When wearing sweat pants or any sort of leggings/yoga pants, make a conscious effort to make sure they're around your hip bones so that, should you sit down, your thong won't show.
Also, for whichever pants I'm wearing, I've developed a habit of frequently pulling them up, even while bending down to prevent my thong from showing by accident.