You get points for:
- Content citizens
- Specialists (which ainwood forgot)
- Happy citizens
- Territory
- Difficulty
- Future tech
Your score is an average of your scores on each turn, meaning that being a tiny, unhappy civ for 95% of the game, and then suddenly booming in size and happiness is not going to earn you a lot of points, because you need to start earning points from the beginning.
Secondly, playing at warlord isn't going to earn you a lot of points either. Its multiplier is 2, meaning for instance you'll get twice as many points (I believe) playing monarch instead.
Thirdly you also get 5 points or so for each technology beyond the modern age you research.
So the formula would be something like:
Turn score = Happy citizens * 2 + Content citizens + Specialists + Tiles within cultural border + 5 * Future techs researched
Summarize the turn scores and then divide by the number of turns. Then multiply with the difficulty.
I'm not 100% sure of where the future techs come into the picture but I think I got it just about right.