Alabama won back to back championships in 1925 and 1926, 1964 and 1965, and 1978 and 1979.
Princeton won back to back in 1869 and 1870, won 4 in a row between 1872 and 1875, won 5 in a row between 1877 and 1881, won 3 in a row between 1884 and 1886, 1893 and 1894, and 1898 and 1899.
Yale won 4 in a row between 1881 and 1884, won 5 in a row between 1891 and 1895, won 3 in a row between 1900 and 1902, and 3 in a row between 1905 and 1907.
Michigan won 4 in a row between 1901 and 1904, in 1932 and 1933, and 1947 and 1948.
Notre Dame won back to back in 1929 and 1930 and 1946 and 1947.
USC won back to back in 1931 and 1932 and in 2003 and 2004 (2004 national Championship was vacated due to NCAA violations).
Pitt won back to back in 1915 and 1916 and 1936 and 1937.
Harvard won back to back in 1898 and 1899.
Oklahoma won back to back in 1955 and 1956 and 1974 and 1975.
Penn won back to back in 1894 and 1895.
Michigan State won back to back in 1951 and 1952 and 1965 and 1966.
Minnesota won 3 in a row between 1934 and 1936 and back to back in 1940 and 1941.
It is important to understand that many of these schools won National Championships on the same years as other schools as there was no single governing body that awarded the National Championship several polls and mathematics's formulas were used to determine the champ prior to 1996. All championships before 1933 were all awarded in 1933 because these were determined on historical research i.e. in 1884 Princeton was 1-1 and Yale was 1-0 neither played the other so both were awarded the National Championship (Harvard went 0-2). To this day there is no single body that awards the title for example in 2003 the USA today awarded the title to LSU and the AP awarded it to USC. However since the inception of the BCS in 1996 no team has won back to back except for USC in 2003 and 2004 but the 2003 title was disputed and the 2004 title was vacated due to the Reggie Bush incident.