Catalytic Converters do deteriorate over time and use. They are not a 'forever' device. You can only control what you can control. Pour-in fuel additives (as others have mentioned), not continuing to drive with a misfire or over-rich condition etc. are about all you can do.
Lead and Sulfur are bad for Cats. We all know that Lead was banned from fuel decades ago. Sulfur is still being reduced from fuel and is at it's lowest levels ever. And because the fuel industry is so heavily regulated for this you will not likely need to worry about it. Except for the added cost of extra refining...
A lot of cars have multiple catalytic converters on them. If you just replaced 2 on your car, its likely there are not any more that will need replacing soon. If the mechanic says otherwise, they are just taking you to the cleaners. (that means they are screwing you over)
Wish you had told us if and what fuel system cleaners you have ever used. Have you ever had to replace any oxygen sensors and what brand of sensors did you use?
don't dump too many additives in your gas, once in a while is ok, but every week is too much
if you don't use additives, then maybe the gas you are buying is full of additives and ruining them.
my car is 22 years old and still has the same one on it and still passes the smog check as if it was a new car
Usually they only deteriorate if there is something wrong with the engine and/or the emission systems that control the engine air:fuel mixture.
If the engine is running too rich, or if it is burning oil or coolant for example, that will certainly destroy a catalyst. Unless you also look after the cause as well, your new catalysts will unfortunately have a short life as well.
In the last 2 years I have replaced 2 Catalytic converters on my Nissan Pathfinder 2006. Is there a way to prevent them from going bad? BTW the 2 converters I replaced were not the same.
