If you are playing the current version then its simple, and I proved it so well that they deleted my score.
I wont spoil it for you, but find a way to hit end turn repeatedly, as the main determiner for score is how long you play.
And galaxy size and number of planets does matter, as the length of time is multiplied by how large your civilization is.
I posted a score of 4.68 million, so I know what I am speaking of.
For my dedication and hard work in exposing thier extremely flawed scoring system, I had my score deleted and all my medals taken away.
For the naysayers:
http://metaverse.galciv2.com/index.aspx?g=player&id=2896
theres the proof i am not lying.
Now, its ok that they treated me like a punk, because you will soon be playing an updated version of the metaverse which will fix the huge gaping problems that I highlighted with that score, and I take the credit for making them fix it. Despite many forum topics about the flaws in the scoring system, they never once mentioned fixing it until I took the number 2 EMPIRE score with one game played, so while I will remain bitter for being treated like a punk instead of given thanks, I still feel good about the change I brought about.
If I am not mistaken the fixes will go live when 1.1 goes live.