There isn't much you can do about it to be honest. I understand what you are saying, I have this crazy freaken stupid idea of .. something in myh ead, but i wont hint or share it because it's what keeps me going and something i want to share with the world. Probably will never happen, but I keep it protected.
As for presenting artwork?
You can't stop theift unfortunatly, but you can create a solid secure network to promote and structuralize your position and connection to your work. So if you become more "known" people will be able to know you are the original creator.
I dont know, the only specific thing I can think of is to hit up every major artwork network at once by createing and linking your accounts together.
Like, Deviantart, Conceptart, even creating a facebook/twitter account specifically for your work, aswell as a website.
Basically use the networks that are there to your advantage and then network those accounts together via your website at the very least.
Besides that... idk, the rest is up to you. Your ideas and presentation are the driving force for securing your originality. Talking to people in the art communities and getting to know them will help, but ofcourse be sensible with what you share. But the more you can talk to other people in these communities, and show them your work, the more you get your name known, aswell as the ideas you are creating.
That's the factual side.
There's the aspect of what you are afraid of, or more so what you want and are afraid of losing. Are you worried that you will put your work online and someone will replicate it or even enhance it and gain the credit and that no matter what you do you wont be able to reclaim that "originality" that you feel now with your artwork, in the eyes of other people?
If you r that obsessed people will still your works just try showing them on restricted circles like blog friends or find a group of equally talented and egomaniac artists that dont copy yet enjoy sharing what they do and appreciate whats others do, so you can focus on the best part of publishing works
show your portfolio as a flash movie. that makes it harder to steal individual images. or set up a password protected site, and ask that people email you for the password. only post low resolution images, and you can always put a watermark on them.
i post my photos online and don't worry about it. they are all low res. the series are in a flash movie. you can look at http://www.marmiteontoast.com
now if you are worried about you plagiarizing someone else, only you can answer that. we all get inspiration from other people's work, esp. when you are starting out. you can always post an about me page that says who your influences are.