Brake.
Also, it's easy: The reason your bike is hard to shift is because you haven't fully pulled in the clutch and the engine is still trying to turn the gearbox while you're shifting. That's because when not using all four fingers on the clutch lever the remaining ones have to go somewhere, and that is inevitably trapped in between the clutch lever and the grip. Which naturally prevents you from pulling the lever all the way.
So don't do that.
You can use fewer fingers on the brake lever if you prefer and I usually do -- this despite what the MSF screeches at everyone. When braking you may need to maintain some throttle control. I rarely use all four fingers on the brake lever.