Practicing with drones is one of the best ways to improve intonation. There are several good resources on the internet for downloading audio files of drones. There are also a number of drone albums available on popular music streaming platforms. Here are a few exercises to play to get the most out of your drone practice:
Playing long tones unaccompanied is great. It helps with control and support. Playing them with a tuner is even better. But, a tuner provides visual feedback, whereas playing along with a drone provides aural feedback. The slightest fluctuation in pitch will produce beats that you must then try to eliminate. When we play in a real-world setting, we have to respond to aural feedback, so this is a more practical approach. Playing with a drone also helps you identify each pitch closer to an exact frequency, as opposed to a range of frequencies. Instead of hearing an A as somewhere between 438 and 442, you will gradually be able to discern even a two to three cent discrepancy.
Pitch bending exercises are valuable for helping to find the center of a given pitch. Doing these exercises with a drone helps with identifying the center more easily, again by trying to eliminate beats. Try starting on a concert F against an F drone. Then try to bend down to an E without moving the slide, or pressing a valve, and then back up to the F. When you hear and feel the F lock in with the drone, you have found the center of the pitch. Pay attention to exactly how your corners and throat feel, and where your tongue is positioned. Then do this on E, Eb, etc., descending chromatically. You can also do this with different intervals against the drone, i.e. playing F against a Bb drone to tune a perfect fifth, or against a Db for a major third.
Scales and arpeggios are a great way to tune different intervals. Each scale degree or chord tone must be tuned relative to the root. Try starting with the F major scale against the F drone. Do not move from one note to the next until you are certain the intervallic relationship is locked in. You will find that you need to make adjustments for almost every scale degree. For example, you will need to lower the major third slightly, or raise the sixth, etc. You will gradually be able to lock in each interval more and more quickly. After doing the major scale, experiment with the natural minor scale, the chromatic scale, and other scales as well. You can do the same thing with arpeggios, and by spelling different triads and other chords.