I have been with zazzle for almost a year and now have two stores. During that time I have experimented with all kinds of ways to maximize my earnings. The most important thing you need to do in order to make significant money is to have plenty of original designs. This is probably common sense to you, but obviously having 1,000 products is going to earn you more than only having only 10. In addition to this I recommend that you update fairly frequently as well.
Another very important thing is that you put on plenty of appropriate tags when you getting ready to put your products in the market place. This is how google finds your stuff and directs people to it, so don’t be afraid to put on as many tags as possible. Also I would recommend setting your royalty amount to a reasonable number. I usually set mine for around 21% and have had some good success. Some people even set their royalties to 15% in order to maximize sales. 15% probably isn’t necessary but I wouldn’t go over 30% and expect to get massive sales.
On top of this, getting traffic is obviously an extremely important part of your success with zazzle. So once you have a quality store set up you need to promote it like crazy. I would start out submitting your store to all the major search engines, meta crawlers, and social bookmarking sites like Digg and Stumbleupon. After this I would join the appropriate top list t-shirt stores at Top Site List Planet. This was a big increase of traffic for me. Also making a lense of Squidoo can help you out quite a bit.