Try as much as possible to put less pressure on your chops when you play. A large amount of pressure will make it difficult for your lips to move from one note to another thus reducing mobility and flexibility. It will also force your embouchure to respond with an opposing force against the mouthpiece and you'll end up wasting valuable energy without knowing it.
Phillip Farkas had a great exercise to practice playing without pressure: he would put his horn on a table and try playing some note using only his embouchure without touching his instrument with his hands.
You probably don't need to go that far! I find that just being aware of how much pressure you are using is enough to notice when it's a bit too much. You can then adjust it to what you find reasonable.
Since I'm the happy father of a beautiful baby girl who was born in june, I haven't had much time to write new music in the past few months but I will soon upload the first few numbers of the Progressive method volume 4. As I wrote earlier, it will be different from the previous books; no more exercise page on the left. There will only be etudes a bit easier than Kopprasch. I find that there isn't much for students at this level so I will try to fill the gap. I'll upload them in groups of 4-5 etudes starting this month hopefully up to 25-30. So stay tuned!
"Sitting on the tempo" is an expression often used by musicians meaning that you take your time with a steady tempo but without dragging.
It allows you to play in a relaxed manner and improves your rhythmical precision. I also find that I have more endurance when I take my time because being relaxed helps me to take more air which sends more oxygen in my blood and helps me playing a bit longer.
Practice with metronome to make sure you don't rush. We don't always notice that we rush when we're used to play in some way. Sometimes we make our life harder than it should be!
I am associate principal horn of the Singapore Symphony Orchestra and the author of the progressive methods. I'm happy to share my experience as a horn player and teacher with you.