The simplest one I've been using is: An upgraded anything is better than something of that in large numbers. My strategy is to build 5 small turrets in strategic positions and upgrade them all the way as well as buying a finance center and gun research center. Then I'll continue with 2 larger turrets and upgrade them all the way. Then I'll buy the laser and the other research center and upgrade them to lvl 5 as well as the gun research center. Then finally, I'll buy a laser, upgrade it all the way (I will leave the research centers at lvl 3 until the laser requires all 3 at lvl 5 to continue upgrading), then build a second one to victory.