Save before trying, then steal everything of value that isn't nailed down. Repair items before trading, try to buy stuff with loot rather than gold, pick up all drops to sell. Buy ale from Rimmer in the dwarven village for 5 gold, and sell it for 50. Have you been to Ars Magicana yet? You can buy mage spell books; cast Resurrect on a monster that always drops something, run away so he turns hostile, then return and kill him repeatedly. At higher levels you could use Heaven's Gift. I never had any problems with money.
If you don't mind cheating, this is safer than the inventory trick. Drag a stack of gold out of your inventory, hold the control key down and drop. When the split items pane comes up, put in a negative number and hit the enter key.
For non-stackable items, you can quick save, drag an item out of your inventory (but do not drop), then quick load and release the mouse button. When the game loads, the original item will be in your inventory, with a duplicate on the ground.
