Practice orchestral parts in the original key instead of using the transposed part. Get all the experience transposing that you can. Which you use might depends on the key or personal preference. This is a lot easier than it sounds. E horn is probably next most common.
Some folks cheat and just read down a half step.
B-flat alto is also useful for filling in second trumpet parts. The trumpet in the band is a transposing instrument in B-flat. This means that a note written for the trumpet will not be the same note on the piano, unlike the violin, flute, and trombone, for example. In this case the trumpet is written one step higher than the pitch on the piano. Other transposing instruments include the horn, clarinets and saxophones.