Harvest and store as much fuel as possible, and keep at least two cargos of it in reserve. If you obtain the technology to harvest stars, you will have no trouble maintaining fuel. If possible, use technology to scan each planet for the resources it has available. When you are well stocked on resources, do not bother harvesting planets with poor resources as you may spend more fuel than you gather. Try limiting the fuel spent to 5 or 6 and if you get a lot of resources, expend the same amount the next time. If not, only use 3 or 4 fuel on your next attempt. Always try to produce more fuel than you waste.