You get progressively more money each time you level up, so gaining experience (by doing quests, exploring, discovering new items, making things etc) will help you toward your goal.
Explore the auctions - see what's fetching high prices.
Just to clarify Rock Opera's statement: You get more money each time you level up until level 20. Then you only get money for leveling up at 25, 30, 40, 50 and 60, I believe. Unless they changed it from what I'd last heard.