We did a lot of work here at Addon Spotlight for 5.0.4 addons, giving you a list of addons that are ready, a survival guide, and a couple of question and answer sessions, which will carry on below.
For Mists, the process is all quite similar. I'll be uploading my list of addons that are ready for Mists, much closer to launch day this time, but I'll also be relying heavily on what I've started to call the "big three" addon sites (WoWInterface, Curse and WoWAce) and their lists of addons which are ready. As I'm sure you can appreciate, there are a lot of addons out there being updated, and, unlike for 5.0.4, these sites are running dedicated, automatically generated lists of Mists-ready addons, and I'm just one girl!
So, is there much you can be doing in advance? If you're a Curse Client user, then a lot of the updating process will be automated, and if you haven't set up WoWInterface's email alert system, now would be the time. Bear in mind that it's a good idea to try to check the addon's home site rather than just blindly following one site's system. Beta files and versions may not appear on each site. Additionally, these "big three" sites are the places where the vast majority of addons are hosted with the authors' permission, meaning they are in line with copyright rules, ethically sound and home to the authors' most recent updates.