@Hinky - Sorry, I know I was going to post up my PM response to share here, but I can't access it now sadly (have posted a seperate help response to try and get it fixed)
But in the mean time, here's a brief summary.
For the Death Eaters specifically, the robes were broken down by hand. The stitching for example (in the embroidery) was unpicked and damaged (thread by thread!:eek) by the costume team.
The Harry Potter costume drama documentary is a great source of info for this aspect in particular.
Video Footage:
By far, this is the best place to start if you want an insight into the various costumes.
ITV - Harry Potter and the Order of the Phoenix - The Costume Drama. (made by "Kindle")
http://www.youtube.com/watch?v=NjKKbxv8iMI part 1
http://www.youtube.com/watch?v=j3nO1Lmt7fw part 2 (check this part of out for the Death Eaters and Voldemort)
http://www.youtube.com/watch?v=zdSrkFNLzrE part 3
TV - Harry Potter and the Order of the Phoenix - Behind the Magic. (Granada Television / Kindle)
- This is quite useful for a bit of general background research and also shows snippets of the costumes so is also a useful starting point but doesn't have as much in depth info as the ones above.
Sadly the Youtube links I had to it are no longer available, but I'm sure a fresh search with the info above would pull up some new ones.
Death Eater's aside, I would say then whenever you're weathering something you should think about:
* what the character is / what do they do generally
* what do they do in that specific outfit i.e. what are they likely to come across during it's use which will weather it / what stresses will it be under, etc)
* the characters personality
* the age of the costume
* whether money, ego, etc are issues
* where the costume has been kept, used, etc
To put this into some context for the Death Eaters for example...
Lucius is a well known, high ranking Death Eater with lots of money and an ego to boot. As a result he'd be proud of his DE outfit, and would ensure it was the best around and always in tip-top condition.
Equally, it's unlikely he'd directly get his hands dirty if he could help it, so it's likely his robes would be less worn than a "lesser" Death Eater.
In contrast, Bella (who is equally as high ranking as Lucius) at one time seems to have some quite intricate robes, but as time has gone on (and since her incarceration in Azkaban), she's become a very dark, twisted character, and so personal image is the last thing on her mind.
As a result, her robes are much more frayed, and although they're not "dirty", the do look like they've haven't been washed for a very long time (as does her hair, nails, etc)
So if you were weathering a Bella costume, you could go much more to town that you would for Lucius
(I hope this is all making sense)
I think there are 2 sides to weathering. If you're going for an accurate recreation of a known character, then it's great to be able to try to recreate that as close as possible.
However, for "generic" characters (or if you're not going for a full on recreation), then I think just getting in the mindset of the character's environment should help you get some pretty good results.
If we can salvage that sent PM (or if you still have it, drop me a copy) I'll try to add anything I may have missed.