Earlier significant tapping of Mexican oil fields, preferably soon after the civil war, coupled with the rise of a socially-minded leader who invests in industry and whatever resources Mexico has. Begin a crusade to stamp out corruption and oligarchism, as well as improve infrastructure, and Mexico should become a pretty nice country.
Another thing this Mexico might do is participate significantly in WWII, utilizing its position as one of the victors to gain favorable economic treaties, especially in Africa. We saw how the war helped America, and while the boost would be less significant in Mexico, it would definitely help.
Then comes industrialization, during which Mexico can slowly shift into a nation driven primarily by service industry. Would that fit your criteria?