Even though United States uses the first to file rule, granting a patent to the first inventor who files a patent application. It is still important to be able to prove that you were the first to invent, meaning the first inventor who conceived and reduced the technology or invention to practice, and that can be proved for example with the date of a working prototype or a well written description. Most countries now use the first to file rule granting a patent and all rights to the first person who files a patent application for an invention. However, if you can still contest someone's rights to a patent if you were the first to invent, and your invention was stolen. It is still a good idea to keep an inventor's logbook.
Keep Good RecordsDetailed records of the concepts, test results, and other information related to making an invention should be kept in a logbook. You can start a logbook from the very first moment you think of an idea. Proper record keeping can be used as proof of the conception date of an invention and to show continuous activity, also refereed to as due diligence. Record your date of original conception then keep a detailed record of your invention activities as you develop your ideas into reality. This is called "reduction to practice' in legalese.
The patent examiner may turn down a patent application if the patent examiner finds an example of prior art that has an earlier date (more than a year) of a filing made by another inventor. Proving the date of your idea's conception and any public disclosure that you made then becomes invaluable - to stop someone from getting a patent if you were not the first to file but first to invent.
What Works - LogbooksThe best way to prove that an idea is yours is by maintaining an inventor's journal or logbook. Read the additional material in the right sidebar to learn how to correctly maintain a logbook. Just like a patent application, if you do not write a logbook correctly you can lose your rights. For example, your logbook should not be a loose-leaf binder; it should be a bound binder. A loose-leaf binder might mean you added pages later and that would make your journal's integrity invalid.
What Does Not Work - Registered LettersDo not think you can mail yourself a letter. Do not hope that your word or the testimony of a close friend will be sufficient.