Thankyou Genghis Kahn for your insightful but wrong suggestions.
To put the record straight...
The year of retirement depends on the difficulty level. If you play on emperor, then the game ends in 2010. The 2100 is in chieftan, I think.
The best way to get a high score is to conquer the world, but keep one enemy city alive so the game doesn't end. Keep the city like a "pet". Block it up with military units and don't let it expand in any way. Now you are free to colonize the world... Cover it with giant cities, get hundreds of future techs, launch a space ship, etc etc. I usually let it conquer the world just to see what my score would have been and then go back to my last save and do this. The score difference is usually about 1000 points.
For example, on an emperor game where conquering the world would have got you 1500 points (depends on how early you conquer the world), that's 150%. Keep playing, however, and earn 2500 points, or 250%. That's a substantial difference.
Further, there is an even better way to get high scores. Follow the instructions on my cheat page to increase the difficulty level beyond pre-programmed levels. This makes the game a lot harder (as hard as you want!) but has the bonus of also increasing your score. I have scored over 350% just for conquering the world on difficulty emperor+2, so there's lots of possibility here for getting huge scores.
The last point you made is actually a good one. Doing this 100% luxuries thing can also push your cities over the edge, into "We love the president" days, which rapidly increase population growth.
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