If you have a piano at hand (elec. Keyboard can work as well). Play a Bb, and then Buzz it (You will be buzzing a C) Work on getting that in tune, trying not to use any pressure. I started on Middle C and went down to the G and Up to C so far, and I play MUCH better in tune now than I ever used to.
A good exercize from the SACHS Book is set Quarter note=60, Free Buzz (lips only, no mpc) on C quarter note, quart Rest, quart not, quart rest, whole note. Then, do it with your mouthpiece, paying really close attention to staying in tune. (Use a tuner if you want, I do), Then, without taking your mouth away from the mouthpiece, put it carefully into the horn, trying not to disturb you embouchure and play it. This will work your muscle memory to that when You play a C your lips go there on command and you only have to change it a little bit to match the ensemble.
After you get C nice and in tune (remember, take your time) Move to D (C on Piano) and do the exercise again, making sure to stay in tune on mpc BEFORE moving onto your horn. Then do the next note, you can go up or down the scale, Whole tone or chromatically. The object is to get your lips to memorize the subtle differences in notes and to just go there on cue without having to adjust to hit the center of the pitch.
My instructor tell me not to spend more than 5 minutes of a practice session doing this, and no more than 10 minutes a day. ( I sometimes do a little more if I am consistently missing out of tune on a pitch.