Old maps!
Have you looked at maps in your research? I love old maps! There is so much you can learn about your ancestors, just by looking up their locations on maps. For example, one set of grandparents lived in Panaca, Lincoln, Nevada...but when they moved there, it wasn't Nevada territory! In 1861 when Nevada Territory was created, it was only half the size of the modern state! Panaca is on the eastern side of the state of Nevada and was actually part of Washington county, Utah when it was established in 1864.
(map from Atlas of Historical County Boundaries)
What difference does this make? A huge amount of difference, when I'm looking for the earliest settlers, I'd need to look in Washington county, UTAH records, instead of Nevada records!
So don't forget to map your ancestors! What have you found out about your ancestors from maps?