I used to run out of cash quite often. Some of my problems were that I bought mercs far too often (and then didn't immedietly use+disband them), and that I used to try to build everything in each province. Once I stopped getting mercs, and began to specialize in provinces, I spent much less money. And when I stopped overbuilding the forts/castles, that helped, too.

Another factor can often be the units used to garrison a province. I used to try to put a stack in each province - financial suicide (and you never really 'grrom' your generals that way). Now I go as cheap as possible for garrisons, especially in provinces that are deep in my own territory, and don't border another faction. I tend to keep two main armies, and retrain these units when needed, instead of building yet another full stack. This gave me the added benifit of increasing the stats of my generals + army units - so now, I can take on much larger armies and beat them down (which just keeps making my own units stats improve that much more).

As far as trade goes, I usually only build trade buildings on coastal provinces early on. Trade is not that big of a factor for me, as I generally get a late jump on shipbuilding. By the time I have one ship in every ocean, some other power (usually one with a better navy) starts trouble, and the trade income is gone, anyway. SO getting most provinces teched up to 40-60% improved farming is what I tend to do.

One last thing. I also now pace my expansion. I tend to play one of the factions bordering the East (loaded with Rebels), so there is a lot of opportunity to grab territory early on. If you take over too much, too fast, the early buildings seem to suck up a lot of the yearly budget. And I tend to look at most of these provinces as cash cows, or just farmland. Some might offer a nice new unit, like Avar Nobles or Steppe Heavy Cav, but for the most part I wont even build a Boyer/Spearmaker there.

Hope these ideas help.