Years of trial and error lol, I've been trying to make a game like this for quite awhile. I wish I could show you a demo I used but I realy don't have anything like that. IMO, the key components are:
-make the player have a top "running" speed that you can get to by holding right/left, as well as a higher top "movement" speed that you can get to by utilizing physics/momentum mechanics
-make the player's jump direction perpendicular to the slope they're on, rather than always straight up
-convert falling speed to running speed when landing
I hope that makes any sense