I am fortunate to have visited many of the 50 states. The majority have been a result of four long drives:

MapLoco


Here is a list of when I first visited each state (to the best of my recollection):

State First Visited State First Visited
Alabama   Montana September 2004
Alaska August 2005 †Nebraska August 1980
Arizona February 1974 †Nevada 1973 (Lake Tahoe)
Arkansas February 1974 †New Hampshire February 1974
†California Resident from 1963-1974 †New Jersey February 1974
†Colorado August 1990 (Resident in 2007-) †New Mexico February 1974
†Connecticut February 1974 †New York December 1968
Delaware February 1974 †North Carolina February 1974
†Florida March 1996 (Disney) North Dakota  
†Georgia (fall) 1986 (business) †Ohio August 1980
Hawaii April 1997 (honeymoon) Oklahoma February 1974
†Idaho August 1990 †Oregon August 1990
†Illinois August 1980 †Pennsylvania (Philadelphia - 1976?)
Indiana August 1980 Rhode Island July 1980 (possibly earlier)
†Iowa August 1980 South Carolina  
Kansas August 1990 South Dakota August 1980
†Kentucky August 1988 †Tennessee February 1974
†Louisiana July 2002 †Texas February 1974
Maine (many times) †Utah August 1990
†Maryland February 1974 †Vermont (many times)
†Massachusetts Resident from 1974-2007 †Virginia February 1974
†Michigan June 1997 †Washington August 1990
†Minnesota August 1980 West Virginia (Chuck's wedding)
Mississippi August 1988 †Wisconsin August 1980
†Missouri August 1988 †Wyoming August 1990

† Have souvenir miniature coffee mug