The bellows for my flat set are quite stiff. They already have over a year of use, so I’m wondering if there is a recommended treatment to help soften the leather.
They must have been made with some awfully heavy leather. There is a leather softener you can get for baseball gloves. Try a sporting goods store.
djm
Check with the maker.
Normally, I’d say neatsfoot, but that may have an adverse reaction if latex or Murphy’s soap was used as a sealant originally.
Practice, practice, practice! 
The pipes may be a year old, but that doesn’t necessarily means there are many hours on the bellows. Seth’s bellows are a bit stiffer than most, but they are utterly bullet-proof.
-Larry
chad i didnt actually use anything just brute force? 9 or 10 hours aday should do it?
Okay, okay! I’lm workin’ it! I’m working’ it!
I’ll apply a large can of elbow oil.
Actually, I am noticing it less the more I play the set. At first it felt hugely different compared to my other bellows and was distracting, but now I"m finding that I’m able to switch back and forth with greater ease. Maybe it’s just my brain that needs to get loosened up.
Chadd, traditionally, brains have been loosened up with Guinness, which, when applied in sufficient quantities, can loosen up other parts of your anatomy, as well.
A good soak in Bushmills can also work wonders.