Even if you're sewing the fanciest evening gown, there is no need to dress up while sewing. In fact, I highly discourage that sort of thing.
In my opinion, the best thing to wear while sewing is whatever you slept in the night before. I'm modeling this look above, which includes my husband's pirate t-shirt and a pair of voile pajama shorts. This ensemble is best paired with fuzzy striped socks.
Even if I come home from work and decide to sew before dinner, I immediately put on whatever I slept in the night before. Even if it has stains on it, as demonstrated above. I highly recommend this route.
The second option is to sew in your underwear! This is very practical, as you can do frequent fittings without a lot of fuss. You should close your curtains first, though.
The third option is to wear a white lab coat (clothing underneath optional), like they do in the Dior couture workrooms. This is perhaps overkill, but it might be fun to pretend that you're a head dressmaker in Paris.
I'm sure I'm missing some possibilities here. So, tell us: what do you wear while you sew?