Obviously, nothing beats experience, especially if you're the sort of raider who learns from doing. It's too late to go back in time and fight the PTR bosses now, but you can make the most of your personal PTR experiences by reviewing the fights you've seen. Take a few minutes to think about what you did on the PTR and what your personal role was; what was hard? What took you a few tries to understand? Maybe you can communicate that to other people in your guild who have a similar role to you but weren't there.
Similarly, encourage your guild to share - and to write down - tactics for the PTR bosses you killed. Chances are you won't be able to field the exact same raid that killed them on the PTR when the patch hits live, and even if you do the poor sods sitting out will need to be clued in at some point. It's much faster for them to read up on your exact strategy than to sit there and have it explained while the rest of the raid taps their metaphorical feet.
Of course, you won't have seen every boss on the PTR and you might not have seen hard modes, or the final versions of the bosses, so there's only so much preparation you can do on this unfinished content. That's where other sources come in!
Videos and Screenshots
Maybe you're the visual type, maybe you just like being nosy about other people's UIs, or maybe you find it easier to get a handle on a fight after seeing multiple perspectives. Either way, there's a host of visual aids you can use to help with your Ulduar preparation.
Ulduar screenshots are ten-a-penny; MMO-Champion has a selection, we've got plenty here at WoW Insider and if you're friends with Google you can find many more. Whether you use these to familiarise yourself with the layout of the instance, to make strategy diagrams, or to create an Ulduar shrine in your bedroom, we're sure you can find what you need.
Videos are less common. You might have caught some of the live streaming during the PTR testing, which was certainly helpful when it comes to bosses on the US/EU PTRs (delete depending on your region). However, a quick search of WCM throws up plenty of videos from Method amongst others. These carry the same disclaimer as the PTR content above - it's obviously not the final version in some cases. However, if you've got time to watch them, these videos will really help you get the feel of encounters you haven't seen before.
I've deliberately not published strategy guides to the Ulduar bosses in Ready Check, partly due to my guild wanting to keep things a little secret, and partly because it's more fun to write about stuff after you've killed it on the live servers. However, there are plenty of people who've gone into great detail on the fights.
Over at StratFu you'll find detailed guides to Hodir, Thorim, the Iron Council, Flame Leviathan, Ignis, Kologarn, XT-002 Destructor and Mimiron. There's even a spiffy map to help you find your way around (nothing's quite as embarrassing as zoning in to an encounter your guildies have been doing for a while, and having to ask which teleport to take..). Some of the guides are more complete than others, depending on the boss, so you might want to supplement the information there.
Elitist Jerks, as always, is a good place to get some information, but be wary of rushing in to post 'so how i kill yoggsaron m8?' or you'll soon find yourself on the wrong side of a ban. The Ulduar PTR Thread is probably the best place to start, unless you're a benefactor.
Let's not forget WoWWiki, a site whose very nature will hopefully ensure it's kept up to date as guilds clear Ulduar. The landing page is a good place to start, especially if you just need a quick rundown of the basic facts about the instance.
Here on WoW Insider, we'll also bring you up to date content on Ulduar; you'll be able to find out more about strategy from this column as well as other posts, so stay tuned if you want the lowdown. Our own Matticus also has some really useful Ulduar strategy info over on his site.
World of Raids has also got some Ulduar guides which include handy embedded videos for you to watch the encounter. Much as a list of boss abilities often does nothing to really tell you what it's like to fight it, sometimes the information is useful especially when planning a new strategy.
And finally, the official forums - that oft-maligned haven for trolls, flamewars and QQing - occasionally have something useful to say. In the case of PTR testing, which was divided between US and EU realms, checking out the forum for the region you're not part of can help shore up the gaps in your boss knowledge. For example, the US forums point us to this guide to Thorim; someone's clearly done their homework.
Channels #elitistjerks and #stratfu on gamesurge plus #mmo-champion on Quakenet are good places to lurk and absorb information. Hang around for a while to get an idea of the local flavour before shooting questions into the ether, though! If IRC isn't to your taste, maybe you can get some useful help on Twitter. I'm going to be monitoring the #ulduar tag, for one.
And on we go...
Hopefully the above resources are enough to get you started on your Ulduar research, if you haven't already compiled a dossier on each boss. Feel free to add your personal favourites in the comments, or to complain that the bosses are made too easy by all this freely-available information.
Good luck with your adventures in Ulduar!