Optimization
There are many nitpicking methods of speeding up your game and making it easier to maintain growth. Sometimes this means exploiting bugs and design flaws.
Trade goods
Unification-Tolerant is a powerful combination, but the use of this bug makes it more so, because of the large production. The bug here is that when a planet finishes a production task, the game gives a monetary bonus as if the planet had been producing "trade goods" for that turn. This can be exploited by producing a trivial item each turn while storing the bulk of each turn's production for later tasks. Trade goods will give you one BC for every two production points a planet generates in a turn, and production can be purchased on sufficiently completed items at 1 production point for every two BC's, so it is cost-effective to refit a frigate to a design of similar cost each turn if you have over 20 production points on the planet in question. In fact, the procedure is cost-effective below the 20 production per turn level iff you are using the money to do things other than buy production. This bug has been fixed in the most recent mac patch by MacSoft, but windows users are left in the cold. Ergo, you should buy a mac.
Delete repeat
Contrary to the game designers' aims, it is possible to purchase production without having to finish the task by putting the task under "repeat", buying it, and deleting the repeat. Despite the indications, the new task you assign will not necessarily be completed in that turn, but if you design a ship to be one turn's production cheaper than the item you want built, it will be completed on time (e.g. some emergency defense). Unlike in CivII, you can only pull this off once per turn. This bug has not been fixed in either version of the game.
Old stuff I haven't updated
Research is the other area wide open to optimization. Often, you will encounter a situation in which the "chance of breakthrough" is charted at 100%. If you hit the Turn button then, you are wasting resources, as research is not stored through breakthroughs. If there is another useful technology which requires just a little more research, you should switch to it, as the previous research points are transferred without penalty. Otherwise, you should place more people into production until the percentage has just hit 100. The percentage required for a breakthrough in a particular field seems to be randomly generated about every four turns, though the frequency seems to change irregularly, so if you are into saving and reloading a lot, you can keep this in mind to save some time. However, it is usually wise to continue some research even at high percentages because of the way the upper bound seems to work.
Some people have suggested selling the starbase and marine barracks in the beginning of a pre-warp game for extra cash (230 total BC's) to use to speed up development. This does help fast-growth races, especially unification-tolerant, which can use the money to build two colony bases by turn 7 (3500.7 in the game - note that you will need to starve your homeworld by at least one food for one turn to pull this off in version 1.6 unless your first colony is ultra-rich). Slower races will have trouble rebuilding the starbase when it is needed. You can draw your own conclusions with regard to selling starbases/battlestations in the advanced game or strategic combat.
The last optimizing strategy I will cover concerns population growth. A fast game usually requires a fast-growing population so more production and more research can be done. The most cost-effective way to grow an empire is to use "housing colonies" which are planets with a single unit of population (set to build housing) and any production-enhancing buildings which can be built. These colonists madly procreate while the main colonies focus on development-related projects, and every time the housing colony's population rises above one, the new population unit is transferred to the main colony or colonies. This is what makes the automated factories technology as important as it is in the early game: a housing colony with automated factories can put out a population unit every 3-5 turns, and this will grow the home planet to full capacity very quickly, enabling large-scale production and rapid colonization. Later in the game, such buildings as cloning center, robo-miner plant, robotic factory, and deep core mine enable the population of a single colony to grow by as much as 2 units per turn, depending on the race attributes (though the benefit of explicit growth bonuses does not apply as much here) and the planet in question. In general, an empire with a large population advantage will win, so a good long-term strategy involves out-housing the opponents, especially with races which tend to colonize quickly, such as unification-tolerant.