You might not believe me, but it helps to sew in just your underwear. I know this after working on a couple of refashioning projects lately.
My first one was the large man's polo shirt with NYPD embroidered on it, which the boy got for me last time he was in New York. Needless to say, it was hugely unflattering because of it's very bigness. I only wore it with over-long bootleg jeans and heeled army-style boots; every time, it reminded me of the time I walked into a party thrown by a lesbian work-friend and realised that the room was completely full of short-haired women in lumberjack shirts.
I spent several hours turning that shirt into one more suitable for woman's body shape, mostly by trying on, cutting, trying on, pinning, trying on, sewing, trying on etc. There was an awful lot of trying on, so most of the time my top half was semi-nekkid.
The other project was to take an ankle-length stretch denim skirt that was too tight at the waist, and turn it into a flippy knee-length skirt. As before, there was an awful lot of trying on, so most of the time my bottom half was nearly bare.
This isn't a method I'd recommend using if your sewing machine sits in front of a large picture window, but it's fine if you have curtains. (Also, I mightn't be so keen in the winter.)
(I haven't quite finished the skirt yet.)