Most Helpful Girl
Wear a cardigan over it, but an oversized/"boyfriend" cardigan. Don't wear shoes that are too dressy - you can try boots (slouchy boots or laceup boots like Doc Martens or something), oxfords, or even sneakers (simple ones like Keds or slip on sneakers like TOMS shoes, not athletic sneakers) if the dress is casual enough. Wearing tights underneath makes it less dressy, and so do scarves, particularly circle scarves or ones that have kind of chunky knitting. Since it's cold out, I like to wear tights, boots, and socks that peek out a little bit higher than my boots.
I've found that if you wear one dressy thing, keep anything else you might wear simple, fabric-wise and color-wise. Don't be too matchy, either. I'm not saying to completely clash, but keep the color palette interesting.
Also, if you don't look too polished with your hair and makeup, it helps make it look less dressy. Like, beachy/messy waves, or a loose topknot or braid in your hair, and light/natural makeup.
I'm practically a pro at this. ;) I wear dresses and skirts fairly often, but I hate being overdressed so I always try to make it more laid back, haha.2