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.
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.