The BERP is a little plastic device you can fix to your lead pipe. It comes in various sizes to fit all brass instruments. You put your mouthpiece in and play as normal. If you don't know your piece well, it'll immediately show so you'll have to make an effort to hear your music better in your head. It's also very good to practice your buzzing. It has an adjustable slide to increase the resistance when you play so it feels easier when you put your mouthpiece back in the lead pipe.
I personally use it quite a lot to build endurance.
You can find it in some music stores or on the BERP website (see link bellow).
Musical Enterprises Berps & Bags
You'll also find some exercises to do on "how to berp".
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.