A trick I've discovered and sure others have as well is to sell cities for techs. Not your cities, but enemy cities. For instance, whenever I am allied with a country against another country, I am able to take a city with my offensive units. Knowing I will not be able to hold that city against an obvious coming force, I back my units on to some nice terrain, sell the city to the civ I am allied with and hopefully have right of passage with in exchange for techs. I am able to keep up with techs in most any deity age this way, and once the enemy takes that undefended city back from my "friend" I simply retake it once again selling it perhaps again until I am certain I can hold it unless I am worried I will lose the city to the allied civ. Works super. Likely my second favorite trick other than using captured workers as bait.