We spend so much time practicing difficult passages that we too often forget to work on basics. The scales, long tones and simple melodies that you were practicing when you started to play your instrument are the foundation of your playing. The stronger these foundations are, the stronger your building will be. If your building a little cabin, you don't need to dig very deep but if you plan on having a 20 stories building, you'll have to make sure you are rock solid down there!
I personally play some very easy beginner etudes sometimes and try to make it sound as good as possible. If I don't do it for a long time, it can be surprisingly difficult but it gets better very quickly. Once I have done that, my whole playing feels better. I also play all my scales every day and spend time to play a few long tones when I warm up to maintain fluidity and a good tone.
Of course you can't spend your whole practicing time doing this. You have a lot of music to prepare after all but do take time to water your roots and you'll find it easier to grow your tree.
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.