For waterproofing leather using modern methods, I've used a few different products.
Nikwax do a water based 'Aqueous Wax' for waterproofing walking boots. This works quite well but needs to be regularly reapplied. As its water-based it allows the leather to retain a degree of breathability.
There are also several wax or oil based products that are excellent - Zamberlan Hydrobloc is one of the best I've used. Again, although its supposed to be for proofing boots, I've used it on leather jackets/jeans/sheaths and it works great. It does reduce the breathability of the leather though (This will happen with all methods of waterproofing)
As for maintaining the leather (conditioning it and keeping it supple). Neatsfoot Oil is good, as is Nikwax's water-based conditioner for walking boots. I give my leather items a going over with one of these products once every 6 months for regularly used items or once a year for things that are used less often.
Give the item a clean with saddlesoap before conditioning and reproofing.
Using this cleaning/conditioning/proofing routine, I have a ten year old heavyweight leather jacket that aside from a few surface scars and scrapes looks as good (if not better) now than it did when it was bought for me. The leather is around 2.5mm thick and is more supple than one of my wool jackets