I brought my long city strap to a leather specialty store because I needed to get extra holes punched. Once that was complete, I asked the man helping me if he had any advice on how to care for the bag (it's my first Bal!) He advised me to take a damp sponge and wipe down the bag once a month in order to avoid the leather drying out. He seemed to be the guy in charge so I assume he knew what he was talking about, but I just wanted to double check here before I do this.
If you look at some of the other stickies in the care and maintenance sub forum there's a ton of info on caring for your Bal bag. (I had a Bal SA tell me to use Lubriderm on my bag and then was told through PF that it's not a good idea as it can harm the leather...so the experts aren't always expert when it comes to care and maintenance!). There are quite a few products recommended if you care to read through the "protecting your Balenciaga" and "moisturizing your Balenciaga" threads.