Do what PDBreske says, get a signature that you can put on all your drawings or make you a watermark and register it.
It is very important for documenting your original works. I have worked many "trademark" or "intellectal property" cases and that makes a big difference for intellectual property rights.
Go back and date all your other drawings, etc. with today's date at the least. If you can document when you created it, do so and date it with that date. Keep all documentation, even the sketches or any other thing you created before the finished product. Sign and date them also.