On an individual level, there are two checklists to run through. First off, your individual resources and self-sufficiency. Consumables are an obvious part of preparing for progress raiding, so stock up on all relevant flasks, potions and food buffs - time to level that Fishing! However, Ulduar's going to be big, and running out of consumables mid-progress is a definite possibility for most people. This is the time when herbs and flasks become obscenely expensive on the auction house; do you have a healthy stockpile of gold to dip into? Or perhaps a herbalist alt that you can use to stock up during downtime now, and if you get spare time during progress. Having an alchemist alt means you don't have to look elsewhere for potion-making services, but friendly guildies rarely charge when it's clear it's for the guild's benefit anyway.
Repair bills also don't come cheap, so ensuring you have enough gold to cover wipe nights, reagent costs etc is paramount. Usually your guild will help you out if you're struggling, but gold isn't hard to get and figuring out which money-making methods work best for you now will save you a headache if you run low on funds while wiping. Remember, you'll need to ensure all your gear is fully enchanted and gemmed as well as being able to enchant and gem new items you might get as you progress through the raid instance, so whether you stockpile gems and scrolls or just gold, you can be prepared.
Your individual skill and performance are also something you can work on while waiting for Ulduar. Now's the time to try out new specs and rotations, familiarise yourself with the aspects of your class you might be less comfortable with (for example, if you're a tanking druid, have you tried healing recently? You may be needed to respec to progress!) and brush up on theorycrafting so you can min-max. It's also a good time to work on your UI and keybindings, try new addons and settings, and generally tweak your play to squeeze the most out of it. Playing alts can be surprisingly helpful here, giving you an insight into how other classes see the encounters you're so familiar with.
As a guild
There are also concerns on a guild level that become important if you're aiming for server (or world) firsts when the new instance comes out. On the organisational side, recruiting new members during farm content is a good way to let them get used to the guild and working with the players in it, although -- as alluded to above -- you can't really see how they cope under pressure. However, you can throw a few challenges at them and see how they respond; can they solo-heal Malygos 10? How do they do on Sarth-10 with three drakes? Can they top the damage meter on Patchwerk? Did they cause you to fail The Immortal?
Since the attitude to raiding changed to "bring the player, not the class" most of the concerns about recruiting simply to ensure a buff is present to min-max progress raiding are no longer valid, but if there are obvious holes in your class (or buff) line-up, they do need to be addressed. You can also spend time working with weaker players and seeing if you can help them improve -- after all, there's nothing better for you to do. Perhaps someone wants to reroll? Better they do it now and gear up, than during progress.
Stocking up on resources also applies to guilds, of course. Selling BoE epics from Naxx is a great way to get extra gold in the guild bank, and having plenty of consumables ready to hand out for tough encounters makes life easier for everyone. Farm-happy members with plenty of spare time now can be amazingly generous, so make sure their efforts are appreciated.
One of the challenges ahead on a guild level is how you will deal with the progress content. Will you jump on the PTR at the earliest available moment and clear it all there, or will you wait until it hits the live servers? Will your members take time off work to get those all-important firsts? Who is in charge of strategies and who will lead the progress raids? How will loot be distributed? How will you deal with players who mysteriously vanish during progress and then suddenly have spare time again once all the bosses are on farm and dropping loot? Who's going to film the videos you release of first kills?
Getting ready for progress raiding does involve work, unless you're already super-rich and have a bank full of flasks (and it's surprising how many people are in this position!). However, the excitement of seeing new content and being among the first to defeat it will more than compensate for the grind you put in to stock up. Hopefully not too much information about Ulduar will be released before it hits live, and even more hopefully, we won't be able to clear it all on the PTR, leaving some element of challenge when it finally goes live.