2008 -- a sitting Senator
2000 -- a sitting governor
1992 -- a sitting governor
1988 -- the incumbent vice president
1980 -- a former governor (who held no elective office for six previous years)
1976 -- a former governor (out of office for two years)
1968 -- a former Senator and vice president (out of office for eight years)
1960 -- a sitting Senator
1952 -- a military officer who had never held elective office
1932 -- a sitting governer
1928 -- the incumbent secretary of commerce, who had never held elective office
1920 -- a sitting senator
1912 -- a sitting governor
1908 -- the incombent secretary of war, who had never held elective office
Everybody else was an incombent ... that includes four vice presidents who succeeded to office when the president died and then were elected on their own (Teddy Roosevelt, Calvin Coolidge, Harry Truman, LBJ) and three who (like Romney) were out of office at the time of their successful presidential run.
I think that data suggests that there is not a formula for successful candidates.
Hmm, let me think ... I wonder what the record of incumbent presidents running for office is (since 1900)?
Off the top of my head, it's 13-4. I've got winners Teddy Roosevelt '04; Wilson '16, Coolidge '24, FDR in '36, '40 and '44, Truman in '48, Eisenhower in '56, LBJ in '64, Nixon in '72, Reagan in '84, Clinton in '96, GW Bush in 2004. I've got losers in Taft '12, Hoover '32, Carter '80 and Bush '92.
Interesting that in at least two ... maybe three of those incumbent losses, there was a strong third-party element involved. The Republicans would have won easily in '12 if Teddy Roosevelt hadn't split the vote with his Bill Moose Party (or not to offend anybody, you could say they would have won easily if the party bosses had not stolen the nomination for TR). Not sure of the exact numbers, but Ross Perot's run in 1992 certainly helped Clinton upset Bush. Not sure if John Anderson's run in '80 had any real impact on Reagan's victory over Carter (even with all of Anderson's 5.7 million votes, Carter would have still finished just short of RR).
Still, the historical precedent would suggest that it's hard to beat a sittiing president in a head to head race. Obviously, it can be done but Romney faces an uphill battle.