I wear an elastic belt outside of my tights or unitard (See other posting in this thread.), and will turn the bottom of the T-shirt up and under the belt, so that all the loose material is under the belt. This makes for a very even line, and the T-shirt is held close to the body, making a smooth line. Some admirerers have thought the combination is actually a specially made-unitard, with lycra tights and a cotton top.
You may have to cut the T-shirt so it's not too long, but don't cut it too short so that it pulls out when you do stretches.
One other thing I do when I have the time is to sew elastic into the bottom of T-shirts (after I've cut them to a comfortable length). You can do this with cropped-tops too. The elastic in the base holds the T-shirt close to the body for a smooth, flowing line. On some T-shirts there is a bottom channel of material...this channel can be opened with a small nick and the elastic placed inside there (but don't forget to sew the two ends of the elastic together!), and the nick sewn shut.
I though what you said at the end was interesting: ">Finally I assume the M. Stevens tights give a very snug fit and that's what I need when I workout because obviously that allows for more freedom of movement so I don't mind if the tights "wedgie" because after all with a dancebelt on there's no problem."
I intentionally "wedgie" the back of my tights into the butt-crack, because I find it more comfortable than when the material pulls across the butt, NOT because I want to show off my great looking butt (Ha! I'll get comments about that!) But I believe this arrangement is more comfortable and allows for more freedom of movement when dancing. Does anyone else out there do the same?
BalletMaster[/i]