See my post at http://www.avvo.com/legal-answers/how-do-i-copyright-my-short-stories--157753.html for information on copyrighting. There are some really good materials at www.copyright.gov that you should review as well.
The more important issue, however, is determining what intellectual property that you want to protect. If you want to protect creative expression (such as your theme designs) copyright is probably a good way to protect that. If there is a logo or some other kind of mark that you use to brand your site, then a trademark is more appropriate. If you're not sure, talk with an IP attorney and figure out what you need.
As you note, your works are automatically copyrighted when you fix them in a tangible medium like the website. You are not required to register each post, but you should indicate on your site that each post is copyrighted with the copyright symbol, the year of creation/publication and your name.
If you do wind up filing suit for infringement you will need to register that work before you can file suit. As you will note in my responses at the link above, there are advantages to registering a work right away that may put you in a better place with respect to compensation. So... you don't need to register every blog post to have a copyright, but need to register to assert your rights.
You asked about proving that someone used content without permission - that is a very good question, and one that involves a fact-intensive analysis. It involves notions of "fair use" - how the work was used, how much of it was used, whether it was a parody of the original work - and involves whether the alleged infringer had access to the original work (copyright does not prevent two people isolated from one another creating the same or similar works). There's much more to the analysis, of course, and if you have any concern that your work has been purloined you should consult with an intellectual property attorney to help you sort things out and plot the best course of action.
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"legal advice." If you need legal advice, consult
a licensed attorney in your state to make sure you
understand both your state and federal laws concerning
your issue. Your attorney will need specific and
complete facts to provide you with legal advice.
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You can copyright your website content. Please see the link below. Your right to compensation for copyright infringement isn't automatic. You have to have your copyrights registered before you can sue, and you have to sue and win, which is way easier said than done and costs a lot.
You can also, and should, trademark your site
You need to see an IP lawyer to know what you're getting in to here.
Disclaimer: Please note that this answer does not constitute legal advice, and should not be relied on, since each state has different laws, each situation is fact specific, and it is impossible to evaluate a legal problem without a comprehensive consultation and review of all the facts and documents at issue. This answer does not create an attorney-client relationship.
One thing to keep in mind in connection with copyright and websites is that while copy and specific images are generally protectable, the "look and feel" of a website is hard to define and or protect. So you should make a sharp distinction between the valuable elements of your blog. Given that it is a blog, your copy is likely what drives the site's value proposition to you and your readers, not the look and feel of it. That being said, you should determine what is the best vehicle for your registration (e.g., a form TX for your copy, as opposed to a more general registration for all the elements of the site.)
You may also want to look at the copyright office circulars below. I hope this helps.
Disclaimer: This answer is for informational purposes only and does not constitute general or specific legal advice, nor create an attorney client relationship.