The most effective part of the learning experience is playing third trombone. I CAN play most of the lead parts now (I do play lead trumpet/flugelhorn/cornet in Jazz and Brass bands) but I am finding the third part requires a high level of reading skill. In a lot of arrangements the third part doesn't necessarily match lead in note to note timing, and often the harmonies don't go where you expect. So even though your ear says to play a certain note at a certain time, what you read is what you have to play! A lot of players will look down on third parts as beneath their "playing ability", but I'm having a ball playing the third as best I can and filling out those gorgeous trombone chords and harmonies. As I said earlier, it's a great way to accelerate your learning curve.
My advice to anyone who is a beginning musician is don't pass up the opportunity to play other than lead parts no matter how well you think you can play. It can be a great deal of musical fun. (I occasionally read a bass part an octave up - great way to solidify intonation!). So to beginning players - join a band if they will have you. New Horizon Bands are ideal for this. Soon you will be contributing to the band's overall sound and you'll feel really good about yourself and your playing. And feeling good about your playing is 90% of the way to becoming a really good player.

K22