If you have an ancestor born in the 1820 to 1850 era, try to determine if he served in the American Civil War.
Civil War pension records contain valuable information that just can't be obtained anywhere else.
NARA site (some libaries have these applications too)
What I found on my 2nd great grandfather is probably pretty typical:
(note: viewing the images using "View - Full Screen" makes them a little easier to see)
Ferdinand Grace's pension declaration from 1923
Foot of his declaration (the docment is too long for my scanner)
Pensioner death notification from 1924
Widow's pension declaration from 1924
Apparently a file page for the National Archives
Bureau of Pensions document from 1894
Claim for pension document from 1924
Death certificate sent to the Bureau of Pensions