Try building more market centers (perferably their upgraded versions). Also, try to use 1 farm and 1 morale building per planet. Those strucutures will increase your taxable population while helping to maintain your people's morale to about where it was before the increase in population.

You might also want to try a different method for setting up your planets. You don't need factories on every planet. A class 6 planet will not have very many tiles to fill up, so it doesn't do you much good to have them built. You're not likely to be able to build a lot of combat ships from that world with its limited selection of tiles, so you might as well replace them with market centers.

You might also wish to play a practice game (maybe at reduced difficulty) where you try to find ways to improve your economy.
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You didn't provide much information, so I'm making guesses to what your problem is. If you still have problems, could you provide more details?

that pretty much what i do tho.. i usually build advance market on planet that safe from harm or next to a factorie planet to protect it. I usually build factories planet near border of other civilization or in middle to protect vulnerable planet

Hi!

1) have at least some military. A fake defender (cargo hull with one cheapest gun, missile and beam will display in orbit 3*2 attack, and so artificially rise your mil score (giving you only one "minus" in relations).
2) have one trade route to them (a "plus" in relations).
3) have better diplo ability than they have (a "plus" in relations).
4) make them fight someone else (a "plus" in relations).
5) play as evil.
6) when they ask for money, give it, if sum is low. If sum's big, deny, but give them immediately thereafter (don't close the screen!) 1 BC for the same effect (works in DA 1.8).
7) in the time you'll buy doing this, develop your planets. Research medium hull and level-2/3 defenses for the main weapon they're using. Build a few most-defense mediums and lots of troop transports. Make them fight even more wars, then start one of your own. Forget their ships (destroy only what you REALLY must), go for their planets and resource-mining starbases, and their econ will crunch on no time (maintenance costs of superdominator corvettes helping a lot there). After that you just mop-up.

BR, Iztok