Then there are the products sold by phone salespeople for big $$. Rich's grandfather bought some years ago, and we swear it was just sawdust and yeast.
We have ours pumped about every two years as well, just to be safe. I remember sometime about 10 years ago when the tank was obviously pretty full, and we had a little burp of the system in the lowest drain. Since then, every two years.
A packet of yeast a month down the drain is supposedly all that's needed. I usually put some down a few days after the odd times we've used Drano or similar products.
Chuck