Your ancestor may have had a stable, conservative and quiet life but that doesn't mean that their siblings did. Was your ancestor's brother famous in his time? This can be the key to helping you go back another generation. And if nothing else, it may give you a feel for the characters of each family member and how they interacted. Looking at brothers and sisters gives you information that you may not have ever found on your own. You should even look at half-brothers and half-sisters.
In early times, men and women stayed married for survival. If one partner died, the other usually remarried. That second marriage usually produced more children, half siblings. President Gerald Ford is one of those famous half-brothers. Ford's Presidential Library clearly documents this for us, but it doesn't mean that you are going to have it documented for your famous relative. Some times you still have to do the research.
Some brothers are famous and well known, like in the case of the Wilbur and Orville Wright. But did you know that they had a sister Katharine? If you were a descendant of Katherine's it would be silly not to look at her brothers information, wouldn't it?
So what are the best sources for looking sideways? Of course, start with the U.S. Census if the family is in the United States. Then look at the area around where they lived. Did they own property? Were they homesteaders? The BLM has land patent records online. The results of a search on the Johnson's in Marion County, MO. confirmed brothers and father. They all went down to the Land office together on the same day. The Patents are in numeric order. I already knew their names. By using the Land Patents I was able to see where everyone lived in relation to each other.
What tricks do you have for gathering information sideways?