Good points from Peace Phoenix. Here are some more
- at the beginning increas the tax rate as high as you can with 100% morale (i.e. approval), decrease the rate when morale gets below 100%
- yes, don't buy anything, build it whenever possible. e.g. start 100% spending by 100% military until you got all the colony ships you need
- after that build the improvements without maintenance first (Soil, Habitat, Banking)
- do not build manufacturing improvements esp. Fusion Power Plant on all planets. Only on the best planets where you want to build wonders, trade goods ...
- don't settle planets unless you can reach a PQ of 15 or higher very fast (use e.g. 100% spending by 100% sozial until you get Soil Enhancement)
- don't research everything by yourself, use diplomacy to exchange with the AI races, before Maso you can even get money for your techs, e.g. you have tech A and trade tech B for it and sell A and B to another AI etc.
- change the government to Democracy quickly