I've actually read a lot about copyrights, though I still might not necessarily know what I'm talking about. Maybe it's different in the UK, but from what I read, technically every original thing you create is copyrighted by you, with some exceptions I would think. If a friend sent you an e-mail and you post it on your website, that's illegal.
I put the copyright in most of my work. My friends are usually impressed that I actually got it copyrighted until I explain to them how it really works. I know that it's hard to prove that it's actually your original work, but at least it would stop people who wouldn't steal it if it's illegal, or intimidate somone who would steal it if it weren't for the copyright.
I also read about parodies. One way you can get away with a parody is if it's critisizing the work. The fair use law allows you to review or critisize works, but if you use more copyrighted stuff than necessary it might still be in favor of the other person. Whether the company lost money due to the parody also affects things, and whether you gained money. If you don't make money from it, you can be sued for as much.
For trademarks, you can use the TM symbol for an unregistered trademark, while the (R) symbol is the registered trademark. Trademarks can be used for things like characters, slogans, logos, and other more vague concepts like that. The trademark only applies for the context it's used in. There could be two companies with the same name apparently if they are different types of companies. You also need to defend your trademark unlike copyrights. If your trademark becomes generic, then you no longer own it. For this reason I think it's ironic that Robitusin actually promotes use of it as bless you for coughs.
In html you have to write © for the copyright symbol. For the trademark symbols you'll have to look it up.
For proof that you're the original creator of a Flash work, having the original .fla files could help, but it's not guarenteed.
In copyrights, you may notice that sometimes they put All rights reserved. It's not necessary anymore, but it used to be something you needed to put so they know all your rights to the copyrighted material are reserved to yourself. Now that's just assumed. I still like putting that though.
By the way, when you submit content to websites make sure you read their terms of service. Newgrounds is fine, and so is imageshack, but putfile isn't. It's very popular, but from what I've read anything you submit to them is basically owned by them.
"Users upload files to Putfile on condition that they release all rights to the image, and give full permission to Putfile and its associates to use and publish the image in any way they wish."