FD,
Having done the sim earlier this year and finding it really difficult, any preparation you can do beforehand helps:
I bought a cockpit layout from
www.flightstore.com it costs quite a bit but definitely useful.
I did my MCC on a Boeing 737-400 which also helped as the cockpit is fairly similar.
I also used the FS2004 software, this has a 747-400 cockpit in it, the speeds and power settings don't work, but it will help you get familiar with the layout of the busy MFDs.
Get familiar with AERAD/AIDU plates, you can get a copy on the Internet from their website.
If you are really keen, you can pay £450 an hour to go in a 747-400 sim with Jetlinx. There is a minimum hire of 2 hours (ie £900), so you may want to find someone else to share.
All that aside, on the day be yourself, the sim staff are very professional and take into account your previous experience. I flew the 747-400 like a biff, yet managed to get through!
Good luck.