dunk it in a bucket full of neatsfoot oil & it will be instantly soft & supple in less than 15 mins of being soaked in there.
voltaire's hercules leather balsam is incredible. it's a little pricey but it makes leather soooo soft. http://shop.voltairedesign.com/US/accueil/66-voltaire-design-hercules-leather-balsam.html
Ko-Cho-Line is your best friend, and one tin lasts for ages. Whatever you get, make sure it's greasy and cream or more of a gel, not oil! Over-oiling your tack can qctually mke problems worse. Conditioners can darken tack though so I would recommend trying it on a small patch somewhere that's not readily visible.
Leather CPR, it's a cleaner and conditioner
Leather conditioners work wonders - proabably use regularly for a while to soften up the leather
The CWD conditioner is really good. Helped brake in my saddle really really well.