I build one farm per planet, no matter what the size. I never build farms on farm bonus tiles, and I never build any farms on 'home' planets where the default max pop is 12b. I let my farms build all the way up to advanced, so my galaxy-wide pop limit will be 13b per planet (12b for 'home' planets). I also build one morale building per planet. Along with racial morale bonuses, this usually lets me keep approval around 100% at about 49-50% taxation once the game takes off.
Also, if a planet is not to be a major industrial or research planet, I just build a couple of factories and a frakload of econ buildings. Most of my PQ 10 and below planets become nothing but big revenue generators, without even a starport.
A population of 13b gives a really good ROI on my econ planets; with one farm and one morale building, the income from the econ buildings is better than it would be if I added another farm (and the several morale buildings it would take to keep the planet happy) or if I built no farms or morale buildings.
Even on my industry/research intense planets, I still build the farm and morale building - not so much for the income, but for the invasion fodder and invasion protection those extra bodies provide. Those can come in very handy, especially if you're caught off guard by a declaration of war.
@jfuqua, yes, you can build new improvements on top of old ones, or just use the demolish option. I always build over excess farms when I capture a planet.
Spore. When you absolutely, positively got to kill every motherfrakker in the galaxy. Accept no substitutes.
