1. If I like it then it's mine.
2. If I saw it first then it's mine.
3. If you show it to me then it's the same as giving it to me; it's mine.
4. If I can take it from you then it's mine.
5. If I had it a little while ago then it's mine.
6. If it looks like mine then it is mine.
7. If I'm doing or building something then all the pieces are mine.
8. If it was yours but you put it down or let go of it or stopped looking at it then it automatically becomes mine.
9. If there are no parents in the room then everything in it is mine.
10. If it's broken then it's yours

I found versions of these online so I don't know who wrote them first, but they are great.  Minor modifications by me.