My tips on designing for Shapeways' porcelain:
1) If it is something you wish to interact with, like a spoon, design it in clay before you start modeling on the computer. If nothing else, this will give you the proper sense of scale while holding it.
2) I had to try, but sprues do not work with porcelain at all. I could not release this design as someone might get hurt. The broken glaze caused sharp edges and the sprues were too strong.
3) You can use a sphere to check the minimum wall thickness while modeling. For porcelain, this is 3mm. So, I created a sphere with a 3mm diameter and placed it around the thinnest edges of my design to make sure the curvature of the design was not sharper than the curvature of the sphere.
4) Cover a plastic prototype in aluminum foil and flatten it down to the surface for testing food safe items. As well as the functionality, the feel of my Bone Spoon was a big deal to me and my first design felt very awkward.
5) Test multiple iterations in a single print to save a lot of time and money.
6) If you make a deep enough hole underneath, Shapeways will glaze inside it.
7) Testing gets expensive, so utilize the cheapest materials while you can.
8) Take advantage of Shapeways' promotional codes. They come up once in a while and can really help out.
9) Try to stay somewhat organized. With all the iterations you may go through, it is easy to get lost. I always end up doing more testing than I thought I would.
10) It is very handy to have the design requirements in mind while you work. Just memorize the most important ones, or use guides to keep checking your work. It's a shame to be working for hours tweaking a design only to discover later that you need to re-work it because it was either too thin, too pointy, too small, or doesn't stand freely.