You can greatly improve your lip trills by practicing them on your mouthpiece. Instead of playing a major second, buzz a major 3rd. So to play a trill from G to A on the horn, practice a "trill" from F sharp to B flat on the mouthpiece. Do 16th notes with metronome slowly in the beginning and speed up gradually. Make sure you play a major third when you buzz; it tends to become a minor third when you speed up!
Once you've done this for a few minutes, put your mouthpiece back on your horn and try a real lip trill. You should an improvement instantly.
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.