I am a believer in buying one size smaller for tucking, unfortunately the smallest panty size I can get isn't small enough. However, I came to a solution to that problem but cutting off the bottom by a few inches and re-stitching it so that it's snug around the waist but tight over the groin.
I'm still a really athletic girl so I tend to use panties for every day stuff, but thongs for a very tight and firm fit for when I'm sparring or wrestling or running.
And I've discovered that thongs tend to elicit a temporary erection. I'm not turned on, or at least I'm not acknowledging it if I am, but obviously stimulated.
Does anyone else have this? I mean, I was curious if it was just my body's natural response to the situation or if it were something mental, like a thong being "erotic" or something on a subconscious level.