@zoltan: Hope I wont sound rude cause It's not what I mean. What your asking as notthing to do with 3D Printer, but rather 3D Design. There's a lot of tutorial on the web for SketchUp or other software. You should make a search about it. You'll learn how to design in 3D, what to do, what to avoid. When it's done, you'll need to search tips for 3D Printer, there will be some tips that will help you think about a design that will print well.
Sorry about this, my english might be bad, I'm french.
Sorry about this, my english might be bad, I'm french.