Hi fizzydumdum,
like Beanylondon said, it's best to hunt down some LMB or Apple products on the internet. Most Balenciaga users seem to swear by these products, both for cleaning and for protecting the leather.
I've just received a selection of Apple products today. I ordered a selection of Apple care, leather cleaner and Apple Garde rain repellent. The products are cheap, each one less than $10, but because leatherstuff only ship using EMS ($45), it pushes the cost up, so make it worth your while.
I've already used some Apple Care on my two city bags. I used quite a lot on my 05 Bordeaux City, which was in desperate need of some TLC. The leather feels extremely supple now. The 06 bone white doesn't really need much attention. I'm going to spray my bags in a little while, then survey the results. At the moment, I don't really think the leather's that well nourished after use, but there seems to be a difference.
I shall order some lovin my bags products later this week as they have the all important For Handles Only and pro-treatment as it's the handles that go first.
Normally I use Russell & Bromley handbag cream on all my expensive leather bags, as the almond oil and beeswax feed the leather, but most Bal owners say that oil and wax shouldn't be let anywhere near BBags. To be honest, in the past, before I heard of Apple and LMB, I used Russell & Bromley handbag cream frequently on my 03 black first and it kept it supple and nourished. If you aren't impressed with the Apple and LMB, I think you should try this product.
All the best, hope this helps x
I'll certainly give that a try, since my pale magenta City looks a bit parched.