yes, preparations before-hand and having a helper is key especially when people are still getting used to SL and the its audio facilities. Perhaps we need to think about competencies for SL-Faciliator (Moderator? or presenter?) and the competencies for the Technical-helper.
I've been thinking about this, as we've had groups in SL; and have found that it's often easy to get sidetracked into helping someone do something. Of course, in a face to face class, students can see that you're helping someone else, and just get on, or put their hands up & wait.
In a virtual setting it's different, as seeing a "sleeping" avatar, or, worse, not seeing the avatar doesn't really let the student know what you're doing.
A second person is a help - though of course if they're busy helping x you have to encourage y to wait..
I've had groups of both staff and students in SL. In both cases it was in a computer lab, so they could see who was getting help, when my avatar fell "asleep". Interestingly, it was the staff who were both more impatient, and more demanding in terms of the help felt they needed to get "sorted out"/ just another question while you're here!
As to what roles the helper needs; I think that it's primarily just a good knowledge of the environment - other students could probably help out, if you've identified those students who are familiar with SL (or whichever environment you're using)