The subtle matter of intonation is largely a psychological one. Each individual player is responsible, and the mere adjustment of slides will not bring perfection. The health and temperament of the players affect the problem, and above all, the aural training he has been given.
The ear plays strange tricks with us sometimes. Some folks possess the ability to pitch perfectly any note you may ask for - a faculty rarely attained by practice, but a natural gift. A musician I know can sing perfectly in tune, yet when he whistles a melody it is always a perfect fifth above - and he doesn't know it! Some folk always sing very slightly flat, or sharp - and indeed some players have such an idiosyncrasy also.
There is no mechanical way of overcoming such difficulties in a band. The only thing to do is to be constantly critical, and as constantly to insist that the players listen to themselves.
Often, when rehearsing a new work, some awkward interval or scalic figure can be made the basis of a short (very short, please!) lesson in ear training.
With an immature band the conductor will have to put up with a lot of bad intonation until the players have learned the notes and rhythm, and can play the piece fairly well together; but as soon as possible he should be at them again about intonation.
Many young and inexperienced conductors get mightily worried about this matter, and their first attempts at tuning a band sometimes cause them acute embarrassment. They must persevere. It takes a lot of practice and experience to be able to stand in front of a band and know immediately what faults are apparent, and what to do about them.
In time one invents all sorts of little tricks to make the players think. 'Up and over that high note, don't strain up to it'; 'Down and under that low note'; "Let that D flat drop towards the C natural' - such are the kind of instructions one develops for one's use.
No text book or series of articles can teach foolproof methods in such matters as intonation. Bandmaster and players must think about it, and constantly purify their listening!
It is not a problem to be left to solve itself. (It is one I would always avoid if I could!) But dealing with it brings ample rewards in the way of purer tone, finer clarity of part-playing, and an improved sense of style by all the players. It is of first importance.