Play on your mouthpiece

I personally practice on my mouthpiece alone every day for a few minutes as part of my warm up routine. I do  one or two octaves glissandos to increase flexibility and fluidity.  It is also very useful to play a difficult passage on the mouthpiece and then play it on the horn. It allows you to focus on the actual movement of your lips and air flow. It's also a good way to find out if you hear the music correctly.