The trick is to PROVE it existed before the patent was filed. And I can tell you: for a patent examiner, it is increadibly frustrating to have to grant a patent, because you can't find a document which shows that the silly subject of the application existed 20 years ago.

But you know what: if you have documents on what you did 20 years ago, you can send them to the USPTO. The people there will put them in their database and use them against future patents.