Using this awesome tutorial I created a new admin menu item with my own entity. When trying to add fields to my entity, I encountered the following problem: new fields just weren't saved. Even when I removed the table and removed the entry from core_resource, it didn't work. Nothing helped until I had the idea to delete the cache - despite all caches being deactivated. One of the cache buttons (sorry, I'm using a german interface and hence I don't know its name in english) finally solved the problem.