You have to keep your approval rating at minimum 35%. Lower than that and your population actually shrinks instead of grow, which means less taxable income. I personally try to keep it around 55% early game and 80% late game.
There are two main reasons for your approval to drop:
1) Taxes are too high - Usually unless you have a lot of entertainment infrastructure you can't raise taxes above 49% without suffering the huge approval hit that tucky mentioned. (I don't believe it's a bug.)
2) The planets have gotten too crowded - If the individual planet has a high population its approval naturally drops. Build entertainment centers to compensate (and research the morale techs). Or build troop transports to ferry people from your overpopulated planets to your not-so-populated ones. Just remember to not to exceed the food capacity of the new planet.
Now, if you're suffering from not-enough-cash so you have to pump taxes really high, there's several solutions to that too:
1) Stop expanding for the time being. Wait several turns. You get tax income based on your planets' populations. The populations start out small but grow .2b per turn until they hit the food cap, unless your approval rating is too low. Stop building Colony Ships for the moment and build Scouts or nothing instead.
2) Convert some of your planets into Economic Planets. Build enough farms to raise your population (I find that 1-2 on a PQ 10 is sufficient) without making things so that you have to replace every other tile with an entertainment center. Fill in the rest of the space with 1 spaceport (optional), 1 manufacturing center (necessary), and the rest Markets or Econ Capitals.
3) Trade. Build Freighters (custom ones with huge amounts of life support) and send them to foreign planets on the other side of the map. Pick their economic planets with huge populations. Then build economic starbases near your planets. Try to cover as much of the trade routes into the starbase's radius as possible, and preferably have 2 or more of your planets also within that radius. Then, as long as you're generating a surplus, construct the production improvements. If you start to get negative cash flow, build the trading modules. Build more starbases and trade modules as necessary.
4) Sell your techs. You can tech like mad and then sell them to the other players to finance your growth. Assuming you don't have problems staying ahead tech-wise.
Congratulations if you finished reading this. Good luck.