J.B.
Hi D.,
A plumber once recommended Bio Clean once a month for our toilets. I couldn't find it in stores, but finally got around to ordering it online. I haven't used it yet, so I can't make an official recommendation, but here's the website I ordered it from with the product info. Good luck!
http://www.masterplumber.net/bio-clean.htm#about
Oh, and I use baking soda and vinegar as well and it kinda works, but this stuff sounds much better and it's still a natural product.