Other people here might have tips too?
If there is one advantage to posting a story from the get-go, that's it. That aside, any kind of evidence is permissible to prove your copyright - witnesses, documents, or even photographs.
The best bet is a history of revisions to show how the story evolved over time, although few keep that types of records, only keeping the latest versions.
Publishing (which posting online is considered as far as copyright goes) is the best proof, especially since the site's owner/publisher can vouch for your ownership of the work. However, once I know that my story is likely to finish, I create an ISBN entry on Bowker (since I ultimately intend to publish). That's considered proof of content ownership, especially since you have the option of posting the actual content for verification purposes. Other than that, filing a copyright claim is another option, though that'll cost you $57 a pop.
However, there's a big difference between being able to PROVE copyright ownership, and being able to DO anything about it. In most cases, the guilty parties aren't typically law-abiding citizen, often residing (or posting) in Eastern-block countries who don't listen to copyright claims from most countries. I had an issue from someone who stole my cover, 'published' copies of my story with nothing but links to other pages (each link clicked fed him page shares, otherwise known as click-bait). He lived right here in the U.S., but without hiring a high-priced copyright attorney to send him a polite cease-and-desist letter, there wasn't much I could do about him.