I wear my tallis katan under my shirt, and I usually tuck my shirt into my pants and pull the tzitzis up to wear they come out the top of my pants and hang down at my beltline, like this.
The only problem is, this scrunches up the tallis katan and gives it some pretty bad creases after a while. I believe according to some poskim these creases change the dimensions of the beged, and potentially make them not kosher (I believe the Alter Rebbe rules this way).
What's a good way to keep the tallis katan straight under the shirt? I tried wearing a "gartel" (an elastic band around my waist over the tallis katan under my shirt) but that didn't help any.
I noticed that a majority of Lubavitch bochurim, they do not tuck in their shirt at all. I was thinking that maybe this was because tucking in the shirt and keeping the tallis katan straight are mutually exclusive. Maybe I'm making up halacha though and there is no concern of creases and folds?