You have to hire the banker in the Drangheim bank, then deposit your money. When you take your money out again (immediately), it has LOTS of interest added. You can keep depositing and withdrawing it and get lots of cash built up. I stopped when I got to around 100,000 gold and that seems to have situated me so that I am not unable to train when I need to.

Of course, also note that I have an apparently corrupted game where I can't promote my druid (kkc was kind enough to edit in her promotion). That's never happened to me before and the only thing I did differently was to hire that banker. Not sure how that would be connected (she's part of the game, after all), but who knows? I would be interested to know if anyone has had the same problem and whether they hired the banker or not.