Trade
Trading is a great way to make money.
Tips & Tricks
For most goods, selling exactly 1 tick worth of goods yields the best results.
For example if a planet requires 374 light ore, selling more than 374 tons of it will earn you far less money than if you sold it just what it needed. There's exceptions to this; things like Hydrogen Gas, Nebula Gas and Water you can stock for many ticks, as a way to get the Upkeep Factor up.
Planet Growth & the Upkeep Factor
Overstocking counts towards the Upkeep Factor. So if you can't find the things a planet wants, (greatly) overstocking it with cheap easily attainable things like or can get that Upkeep Factor above 1.0.
As long as a planet's Upkeep Factor is above 0.5 it will recieve an increase in population, though the maximum population gain is achieved at Upkeep Factor 1.0, you can raise it further than 1.0.
At higher populations, an overcrowding becomes an issue. Larger planets will grow less, and will eventually stop growing.
There is also a random factor in the planet growth formula, so you could see some unexpected growth patterns if you pay attention.
References
The Planet list may be useful for helping you find the closest planet of a certain class, or for seeing the planet-type balance of each cluster.