From: Ontario, Canada
If you start your game with only one colony (this will not work as well if you start with more than one colony) you can drop your taxes to 0% to encourage population growth. This will also increase the planet value and GDP (GDP is what you are taxing). When the planet hits maximum population, throw on some taxes and you'll be making money (cashflow) hand over fist!
While your planet is growing, you'll likely experience negative values in both Money and Cashflow. Just bear with it while the population grows. If you end up needing cash for something, temporarily put your taxes back up until you get the Money you need then drop the taxes again until population is maxed out. (Your mileage may vary on this point depending on what you need).
As an added bonus, if your cashflow and money are at minimal (or better yet - negative) levels at the beginning of the game, the Pirates won't ask for as much money for protection. Instead of paying them 8K - 20K for protection, you'll only pay 3-4K depending on your money levels.
Perhaps I'll just link you to a playlist I did on for both the above points:
One more thing to watch out for is inflation. Check that all your strategic resources (in the Expansion Planner) are valued at 0.8 - if you see any resources more expensive than that (2.5 is the highest) then try to mine more of them with additional mining stations. More mines will only help the situation, not cure it. Sometimes you just have to wait for the supply/demand to sort itself out before you'll see those 2.5's drop.
So - for example, if Steel is priced at 2.5, then your ships and bases are going to be pretty much TRIPLE the normal cost in both purchasing and maintenance (everything uses TONS of Steel). If you have a fair amount of military this will cut into your cashflow quite heavily.
As far as negative Cashflow itself? It's a bad thing, but not crippling if you have a good amount of trade agreements, space port income (from Private Sector purchasing ships) and resort bases. Granted, these help out more a bit later in the game, but start working on them ASAP.
Hehe.. my current LP on YouTube I was running more than -900,000 in cashflow but was rockin' in the other areas. Still lots of money on hand so I'm still able to function quite nicely.
Anyway, DW is a lot about backward engineering. Try things out, experiment. Keeps what works, toss the rest. :)