Try to get to the site a few hours early even if you don't think you have much to do there ahead of time. The idea is to prevent last-minute surprise problems and to be relaxed when participants start to arrive.
Ask the registrar to arrive early too. Welcoming and directional signs should be set up and the sign-in table ready before anyone else appears. Be prepared that some people may want to change their room assignments. Try to accommodate people who will have difficulty walking, or climbing steps, by assigning them bed space as close to the meeting space(s) as possible.
As soon as you arrive, check in with site manager(s) and find out if there are things you need to do or to announce to the group. (It's a nice touch to introduce site manager(s) to the group early in the retreat, for welcome and announcements. Sometimes site managers are interested in being part of various group activities.)
Check out your meeting space:
Set up chairs as needed.
Learn from site manager(s) where all light switches are and how to operate heating or air conditioning and PA systems.
Put up posters near registration table and one or two other spots with detailed time schedule, to supplement handouts. This will save you from answering the same questions many times over!
If your program involves other people taking parts, such as lighting candles in a ritual, or doing a reading, begin to line those people up as they arrive. If you are asking someone to read something, it is a special kindness to give them a copy to read over and become familiar with well in advance.