Help with Slowing down an Entity Moving Uphill

I'm working on a skiing/snowboarding mod and the one piece I'm having trouble with is having my skis (living entity) slow down upon an increase in incline. Similar to how minecarts slow down going uphill without power. I'm willing to do a touch of coding but I have no idea where to start with that.

On a side note - Anyone know if it's possible to mount an entity standing up?

Thanks!