Minecraft wiki entry for Multiplayer_PvP_bases, associated text: Stockpile diamonds and iron, as well as setting up an overworld outpost
Improve defenses, as players here are almost guaranteed to be equipped and experienced.
Set up a blaze grinder, as well as one for wither skeletons
Hoard nether quartz and nether wart if at all possible
Run fire drills with your faction, as these will be much needed
Set up an armory, with at least 2 designated sets of gear for each faction member
Stock up on fire protection potions, fire protection enchantments, and anything else that would prove useful against flames.
Hoard everything you can get a hold of, you NEVER know what will assist you in the nether.
Expand your melon farm, and perhaps set up a tree farm or mushroom farm