Four things that have helped greatly:

1) I keep a concert F drone playing through our Remington warm ups. When playing a three note set (i.e. Concert F-E-F), I emphasize that the last note must sound exactly like the first note.

2) I have brass buzz their Remington warmups at least twice per week while the woodwinds play

3) The last thing we do in warmups is Concert F around the room by sections every day.

4) At least twice per week, during our Remington warm up, we will play three notes (i.e. Concert F - E - F), then sing them

Once I instituted these, our intonation has improved a lot. Judges rarely mention issues at C/SR