This is actually a complex question the answer to which is not totally known.
My basic guess is that your score is essentially the area under four curves shown under the Timeline tab of your Civilization Manager. The important curves are Economy, Military, Population and Research. These pretty much directly correlate into your Economy, Military, Society and Technology score (again I *think*). Really it's pretty clear for everything except the Society score. Population is definitely a major portion of the Society score but your Influence may also play into it, I'm not really sure, perhaps even social production comes into play as well.
One thing I am sure about is how the four components of score (Economy, Military, Society and Technology) contribute to your total score. One thing that used to mystify me is that the sum of the four components of score does not equal the total score. Also if you look at different folks scores including the four components you'll notice a very strange thing and that is that the "better" players will generally have a total score that exceeds the sum of the four components of score whereas the "weaker" players will have a total score that is far less than the sum of the four components of score.
All of this is supposition but it appears that each of the four component scores are "raw" scores and the total score is actually the integral of the four component scores each divided by some function of the turn number. This is very important. An extra 10 bc's of income earned in turn 1 are 100 times as valuable to your score as 10 bc's of income earned in turn 100. Put another way that 10 bc's of income on turn 1 are worth 1000 bc's of income on turn 100.
Basically to maximize score you need to maximize the four components of score; income, population, military and technology. Also the ealier in the game you can get any one (or more than one) component to a high level and keep it at a high level for a long period of time the higher your score will be. Income and population are obvious. Military is merely your military ranking that AFAIK is attack points plus defense points plus hit points of all ships. Note that all bonuses apply including bonuses from resource mining, military starbases (a big source of score) and even the Spin Control Center. Technology is simply money spent on research. How many techs researched doesn't matter. Or even if you are researching techs at all doesn't matter. If you keep spending on research after the tech tree is complete that still counts.
Finally the point is what kinds of values of score can be achieved from each of the four components. Basically, technology is a dead end. The values that can be achieved via Tech spending are very low compared to other areas and you are far better off ignoring it completely. I've tried building many multiple uber research planets each producing 20K RP's per week, totaling well over 200K RP's per week and gotten hardly anything out of my tech score.
Population is a good source but there's really not too much you can do about since the nerfing of the morale buildings. Basically getting all your planets into the 13B pop range with perhaps 1/4 of the top PQ planets to 20B is about all you can do. However where there is a big source of differentiation is in how early in the game you get your pop to the highest possible level.
Income and military is really where you can score big, particularly with the military. Building an array of military SB's and parking a whole lot of ships under it is key.
All of these thing are generally well known throughout the community. There really are no secrets, but what really separates the men from the boys so to speak is how early in the game can you accomplish these things. Even as recently as about 6 months ago I would look at my scores and wonder why they were so much lower than the top players. It was really frustrating because I knew I was doing everything that they did yet still they consistently outscored me. The key is that although I did the same things that they did, they simply did them far faster than I did and that was the difference.
Anyway as I said a lot of this is guesswork on my part and for what it's worth I hope it helps you.