Top tips are:
1. Keep your kit dry.
2. Never ever ever use an anti blister lubricant on your feet, despite advice on the walking about link posted above!
3. Check the weather:
http://www.metoffice.gov.uk/loutdoor/mountainsafety/brecon.html
4. Heed Spenny's advice, Pen Y Fan is lethal in high winds- which are forecast for Monday/ Tuesday- Do not walk on the well worn path along its edge- take a couple of paces in, it may be boggy but it's much safer. The path is well worn by tourists in the Summer months!
5. Be aware visibility will probably be poor early next week so good navigation skills are essential unless you want to walk 30K on day 1 instead of the 18k I expect you need to walk.
6. Are you sure it's the Brecon Beacons or could it be the Black Mountain Region it certainly used to be?
7. Enjoy it and don't take risks, know your escape routes, there are plenty. Remember if you twist your knee or ankle you are going to have to return and do it all over again! Keep an eye on each other, it's a team event not a competition, shall I stress that some more think TEAM.