I respectfully disagree with some of these posts about economy being the #1 priority. In my current game with 9 Civs + 8 minor races on a large galaxy I have been running a negative economy since the beginning of the game and doing pretty well.
How do I do this? By selling technology. Selling technology brings in ridiculous cash. It almost seems like I'm cheating. This strategy only gets easier the more AI players there are. Once you research a tech that no one else has you can then turn around and sell it to every single AI player out there for 400+ cash. One technology can bring in thousands of bc.
Also, I give away technologies for economic/technology treaties. This is huge as it more than pays for itself in a short amount of time. For me, utilizing technologies to acquire agreements from the AI has been the key. I get them to declare war on each other etc.
Granted I am still a newbie in this game, but I've found money from trade to be insignificant. I can get maybe a couple hundred bc a turn from all the trade routes? Pfffft... Who cares? Sure this adds up over time, but it is nothing compared to selling technology. Of course I keep my unique weapons/defense line to myself, but I sell the others that they will be researching anyway. I have found it difficult if not impossible to get into a positive economy until later in the game when all planets get fully developed with population/banks.
Your tips to not build on planets early on is something I will have to look at as I hadn't even considered that option. I have quite a number of questions about the game regarding the inflow of resources which don't seem to add up, but I'll start a new thread for that sometime.
BTW I play on the tough level which I think is the hardest level the AI plays at without cheating.