ebay..

[mrs maggot]

If you have created a unique design, you can print off hard copies and send them to yourself via registered post, (don't open the letter when you receive it) this will be dated and can be used to prove its your design

but you would also need to number the envelope to know which design is inside, as once opened - whats to say you had not put a "new design" in an old envelope - hence why you should also ask the post office to date stamp the drawing before you put it in the envelope, its a bit of an urban myth, but its better than nothing,