-prioritize. Pick the single most important thing you need at the moment and get it at all costs, once it is achieved you move on to the next goal. Example: don't try to build wonders and infrastructure and win a war all at the same time. You'll probably stalemate the war, or worse, lose it, not get the wonder, and not get infrastructure up. Do one thing at a time.
Well, this thread has been going long enough to repeat a few, but... always call the other civs "the enemy". The AI doesn't know that, which is why they'll let you build a spaceship without DoWing you. But you should know it. You might trick the enemy to obey you, to be on your side; but in the long run, they are the enemy.
They're only an enemy if they have the capacity to win before you do. Otherwise, they're a valuable resource that can give you a leg up over your true rivals by serving as trade partner, distraction, or buffer zone. This kind of leads into my tip, which is . .
229. Vassals are awesome, so only take worthwhile cities when conquering your opponents. Leave them the ice & tundra fringe (gaining you access to those resources without paying maintenance on marginal sites), or those cities that will be rendered useless by culture swamp if you conquer them yourself (leaving those borders intact gives you a great staging ground towards your next opponent). Especially on Deity, a vassal with a few marginal cities can be very, very useful. Direct their research to secondary techs, feed them all of your priority stuff in exchange, then watch as they mass upgrade their vintage military; the stronger they are, the better. Just make sure you get the juicy core cities for yourself!
229. Vassals are awesome, so only take worthwhile cities when conquering your opponents. Leave them the ice & tundra fringe (gaining you access to those resources without paying maintenance on marginal sites), or those cities that will be rendered useless by culture swamp if you conquer them yourself (leaving those borders intact gives you a great staging ground towards your next opponent). Especially on Deity, a vassal with a few marginal cities can be very, very useful. Direct their research to secondary techs, feed them all of your priority stuff in exchange, then watch as they mass upgrade their vintage military; the stronger they are, the better. Just make sure you get the juicy core cities for yourself!