Keep two fingers on the clutch at all times. If it starts bogging out on a hill or coming out of a curve, keep on the throttle and feather the clutch a couple times. This will get the RPM's back up and get you right back in the power very quickly. Clutch on the downhills, you want to avoid using the engine braking that the 2t doesn't have much of anyway as it can oil starve it and cause damage. If you're coasting to a stop from a high speed run in an open space, do not downshift through the gears to slow the bike up. Just pull the clutch and use the brakes to slow/stop.
The 125 will need a lot of clutch work compared to the 4t's you're coming off of. It's all good, it will teach you more technique and help you become a better rider.