Brian Moore wrote on a refereeing forum a few years ago about managing the scrum.  He has a lot of opinions on the matter.  He also knows what he's talking about.  Somebody compiled the things he said into a post on RugbyRefs.com.  I've reprinted the text below.

Some of the information is a little dated because the laws have changed, but the principles remain the same.

Managing the Scrum – Tips by Brian Moore (ex England International Rugby Hooker)Moore represented England, winning a total of 64 England caps between 1987 and 1995, making him the 13th most-capped Englishman (as of July 2007). Moore played in three Rugby World Cups including in 1991 where along with Jason Leonard and Jeff Probyn he was part of a destructive English front row as they reached the final, losing a tight match 12-6 to Australia at Twickenham. Moore was also a member of the England side which won Grand Slams in 1991, 1992 and 1995. In 1991, he was voted Rugby World Player of the Year, a decade before the sport's governing body (the IRB), began its awards programme. He went on two British and Irish Lions tours, winning five test caps. In Australia in 1989, the Lions won the series 2-1, and Moore was famously caught celebrating the morning after on Sydney Harbour Bridge, doing aeroplane impressions. Having been a vocal critic of referees for many years. Moore took the Rugby Football Union’s Entry Level Referee Award course and qualified as a referee in 2010. (http://en.wikipedia.org/wiki/Brian_Moore_(rugby_union)) The following are tips by Brian Moore in managing the scrum, these tips were extracted from a rugby refereeing forum and was submitted by Brian under the alias/user BCM666. In this forum BCM666 (Brian) says “anything I post can be disseminated to anyone; including Paddy O'Brien!” so it is with this general exclaimer that the following document is produced.Pre-game brief about scrum

1. Before the game talk to the scrumhalf and the front row whilst the team is in the changing room. This is so that their peers know what they have been told and they cannot claim afterwards that you did not tell them something. It also helps you on the field because if they give away a number of penalties or free kicks it is highly likely that someone will say "He told you about this before the game - why don’t you listen"
2. I would tell them that whatever has happened previously in this game the ball is going to go in straight and by that I mean down the middle line which is below where the shoulders meet, not anywhere else. Furthermore, if this doesn't happen I will yellow card without hesitation.
3. To the front rows I would say that I will make a long mark where the engagement has to take place and they should use it because from the mark I will judging whether they or the
   

The TRRA does good webinars pretty frequently and posts them on their site.   A couple of them are unavailable when I click on them for some reason, but the content is available for most of them.

Topics include:

• Referee Development Through Self Assessment
  
• Tackling the Breakdown
  
• Setting the Scrum
  
• Assistant Refereeing
  
• Effective Communication
  
• Technical Zone & Assistant Refereeing
  

England rugby has a referee resources page with some online courses about the scrum and the tackle and about preparing for whatever their first referee course is. 

The USAR referees resources page also has a bunch of training videos like the one I posted yesterday on various topics.  At least some of them seem to also be available on Vimeo.  Let's see how many of them I can put on this post before Weebly complains.

 OK, let's start with Mike Cobb's scrum video from 2014.  Cobber walks you through some stuff about the scrum setup and then shows some video clips of scrums gone bad with commentary about why.  It's a really good intro to refereeing the scrum.