Ask For What You Want
In hindsight, I realize the biggest problem was that I was afraid to take a real leadership role in the music making. Whenever you are the soloist, whether it be with piano, band, or orchestra, you have to proactively take a leadership role. This means knowing the score, knowing what you want, and knowing how to ask for what you want. If the pianist is moving the tempo in a way you don't want, be vigilant about asking for it to be different. If the band or orchestra is playing too loud and you are being forced to play the piece louder than you would like, demand that the ensemble play softer. Players who are uncomfortable making these demands end up having compromised performances. I know this is the case because I've been there myself.
By comparison to how often we play in an ensemble, it is a rare opportunity for a trombonist to have the spotlight and be a soloist. Take control of the situation and feel free to ask for what you want. If you ask nicely and your musical convictions are sensible, most people will go to great lengths to make sure you get what you're asking for. There's nothing more frustrating than working up a recital or concerto for months only to have it derailed at the eleventh hour by an accompaniment that seems out of your control!