Census and probate records are also very helpful. Some census' contain more detailed information than others. In the U.S. they go back to 1790 when Head of Household enumeration was taken. Probate records are one of the most underutilized records that are available to us. Often wills contain information regard siblings, spouses, children and even their children's spouses.
Some counties have their older probate records indexed making it very easy to conduct searches.
All the advice I have given so far is working backwards from yourself. When you hit dead ends, as we often do, sometimes working forward can lead to useful insights. By working forward I mean establishing what I know about other Buchanan families and try and work forward to see if you can obtain circumstantial evidence link your family to a known line. There are many sources available to us on Buchanan genealogy to assist us in this endeavor. This book that is considered to be the 'bible' of Buchanan genealogy is.