Well i'm kind of a newbie but i have a few tips i do, i doubt these work very well in low tech games but i'm in a 3 planet start, high resources, high tech game against High difficulty ai with high bonuses, this is standard vanilla seiv gold. In the first few turns i colonise the best planets, in between any value from 70%+ Just say it has 120% in radioactives i'd build 2 radioactives resources and then a radioactives collider and then fill the planet up on radioactives extractions. Then once i have a stable resource base colonise any planet and build atmosphere modification plants and any planets that have less than 70% i build intelligence planets on them, filling them up with intelligence centres then half way through the build queue i add the two intelligence modifier facilities (can't rememeber their names) and then fill the rest of the planet up with intelligence, on the atmosphere modification worlds i add a 5 resource storage facilities then add more facilities once it's converted and i have a use for it
I'm up to turn 96 in a high tech high bonus game and i have 50 planets, over 15 systems, i havn't colonised that many systems totally because my ally has taken the rest of them up
But after you got enough defense ships and resources i suggest you colonise the rest of your systems. My profits per turn are +270497 minerals, +232207 organics and +386729 radioactives and i have 51 ships, 3/4's of which are dreadnoughts and baseships.
This is my style of play and i'm a newbie and havn't read any tips or stratergy guides, this is just my way of playing i guess so i don't know if it's effective
Keep playing and i hope things work out for you, hopefully these tips will help you out in high tech games, and i suggest doing this in low tech games except perhaps adding research facilities to most planets along with resource extraction facilities, and then when you have the atmospheric modification facility run wile with that, but i doubt it will be used that much in low tech, i havn't played many low tech games and i have came around low high to mid table and then got wiped out in them
Keep playing and you'll find out a way round it, then you will develop your own way of playing it, your own stratergies and other methods, it's all about practice i guess