For my day job I do a lot of period illustrations (1930s-1940s mostly). I have (on loan) a battered primer from the 30s that I use to get an idea of the clothing and I also rely on blogs dedicated to old advertisements and photos. Beauty Is A Thing of the Past has all kinds of great photos from hair dressers magazines that are fantastic in figuring out how to style hair. And Found In Mom's Basement is a terrific source of vintage advertising. I've learned a lot doing these illustrations.
I've also done a ton of Amish illustrations. If you're ever up in Shipshewana, IN you can look out for the Childhood Chuckles, Farmyard Funnies & Thoughts with Thelma book series. The first two are collections of stories sent in by (mostly) Amish people and the Thelma series is a series of stories by a local Amish wife and mother who lives on a diary farm.
All of this is great fun and I adore it. But sometimes you just want to draw something modern.