The most ironclad protection comes from actually registering your work with the federal copyright office. Doing this requires some paperwork and a fee, so it might not be worth doing for a lot of your work. If you are writing blog articles, and the words are not tied to the value of your product or service, common-law copyright is fine. To do that, all you need to do it to add the copyright symbol “©” on the website or printed material. A good practice is to also document “first use” of that writing. A good way to do that is to make a copy of it and physically mail it to yourself. The date stamp from the post office will document the date mailed and first use. Make a note on the outside of the envelope as to what is inside. Put that document in a file in case you ever need to prove that you were the first to publish that information. We design postcards and we always send ourselves one and keep it on file.
For website content, you can also do a screen-shot that shows time and date, and save that file somewhere secure.
The reason this is important is that should a copyright dispute come up, the first thing you will need to prove is that you created the document before anyone else. If what you created was truly original, then you should have no issue proving that fact.
Now if you are creating a work of art, or a song, or a book that is going to be sold for profit, then registering for official copyright is probably a good idea.
Trademarking is a different animal. There is no such thing as a common law trademark. You are going to have to go through the legal process to acquire a trademark symbol for use with your trademark. To do that is a fairly lengthy and complex process and is best handled by an attorney specializing in trademark law. My experience is that it costs about $1,800 to $2,500, and about a year to apply and have a trademark approved. And there is no guarantee it will be approved. There are many rules associated with trademarks, and there is also a dispute period where anyone can dispute your filing. It can also be disputed even after the trademark is awarded, but it is harder to win the dispute at that point.
We recently filed a trademark to use the term Streetview Mailers for a product that we had created that allowed us to pull down the Google Street View image and print it onto a mailer. Although I know that I was the originator of that idea, and the trademark office approved the trademark, during the 60-day dispute window, Google disputed it. They said they owned the trademark on the word Streetview. We ended up cutting a deal with Google to release the trademark to them. We could have chosen to fight them in court, but they have a lot more money than we do, so we elected to make a deal and move on and create a different name that has been trademarked successfully since.
When it comes to disputes, most of the time, if you can prove that you are the originator of an original work, even with common-law copyright, a simple cease and desist letter from an attorney will get the desired result of the person copying you to back down. No one likes to go to court except the lawyers.