Don't Overfill The Funnel
Imagine you are putting oil into a car. You have the bottle of oil, the funnel that you sit into the tank, and the tank itself. For our purposes, your lungs are the bottle of oil (filled with oil/air), the funnel is your oral cavity with the releasing end being your aperture, and your mouthpiece/instrument is the tank. Essentially, the back-pressure is created when the amount of air put into the funnel is too much for the funnel to hold. If you are pouring oil into a funnel with a small opening at the bottom at a very fast rate, eventually that funnel will overflow. You are left with two options. You can either pour less oil or you can make the opening at the bottom of the funnel larger so more oil can goes into the tank instead of overflowing. As that relates to playing the trombone, if you are having issues with back-pressure, experiment with using less air or increasing the size of the aperture. Seeing as how most players have the issue of not using enough air, I would start with opening the aperture to allow the air that you are blowing to actually get past the lips and into the instrument.