Unfortunately, "yelling" is often necessary because you have to make yourself heard above the ambient noise from the crowd and competition. It's important to learn how to be heard without being aggressive. The other thing to understand is that the on-field portion of drive coaching is only a part of the responsibilities of a drive coach. Working with your partners off the field all the way up to the teleop bell is as important, if not more so, to ensuring success on the field.
In addition to the excellent Simbot Seminar Ether linked above, the next
Behind The Lines segment is all about effective team communication, including drive coaching. It's airing live this Wednesday (11/19) at 8pm ET/5pm PT with Andy Baker (WFA and 3940 mentor) and Mike Corsetto (1678 drive coach).
The last half of the show is live Q&A, so tune in and ask questions!