Ok thanks for all of the replies on this topic. There's no clear solution here, so it's going to be a “Greatest Hits” of the thread!
First off, we have an article on the wiki about announcements to get some ideas,
http://wiki.internationalmagicjudges.net/index.php?title=Tournament_AnnouncementsNow, props to Joel Bouzaglou for being the first to reply with some of the great basics we want to include. Things like, well, his post!
“I think (in no particular order) you may want to state what the rel of the tournament is, how many people you have attending,how many rounds you'll be doing, a brief overview of any recent rules changes, thanking the TO for the tournament, tell the players of any parking restrictions, introduce yourself to the players, explain to the players how to make a judge call, where to turn in the match slips (if any), and then give them permission to start opening their guild packs. That's everything I can think of for now, feel free to add more! ”
Additional Props to Josh Skura for the reminder that the DGM prerelease promo is NOT playable in the prerelease, that will be an important reminder at your events!
Emilien Wild came up with this pre-announcement idea that I've never heard of, but would be interested in seeing how it works. I like how it gives the new players a time to come and learn about the tournament structure without having to make your announcements for all of the players needlessly long.
Mark McGovern has a great idea here, something that adds fun to the deckbuilding process:
“At a prerelease, we encourage you to ask your neighbours for help - especially if they're your guildies!”
This highlights one of the MOST IMPORTANT aspects of head judge announcements at regular REL, especially a prerelease. Keep them FUN! The whole point of a prerelease is that it should be the most laid back and fun event for players both new and old and Mark's idea hits that on the head, bravo!
Martha Lufkin has another great idea and this is something I do at my events, cutting the announcements into 2 sections. Players are eager to bust packs and get to griping about bad rares or high fiving about mythics, so keep your initial announcements short and let them get to it! Talk about the more mundane stuff after they've built their decks and are ready to play. This keeps the players engaged and lets them do what they came to do, bust packs and have fun!
Finally Jasper has a really good reminder, especially if you have a lot of new players. It's easy to see that with encouraging players to help each other build their decks that they might think they can trade cards. Make it pretty clear that this is not cool and do so before deck construction.
In summation, this was a great topic with some great replies. I don't have much to add to the wisdom already shared in here except to keep it brief and keep it fun! Stay tuned for the next scenario from the Knowledge Pool!