Should be noted features like Microsoft Word would likely be the safest method. You can save it to multiple locations (Hard Drive, USB) and guarantee you always have copies with you. So if something ever happened to the site and/or server you'd be safe.
However, these copies would then lack the code of the site in question so if you're applying image codes, fancy borders, spoilers etc it wouldn't give you the end appearance to make a judgement on. Which is where I'd start looking at the following:
This is what would work to keep it on-code and to make sure no one else see's it if you value privacy for your work/collection.
Like PM, but opens it up to the public eye so you can get feedback. However you can also set privacy here to be only those that you follow, so it's still for the most part private (or completely private if you just follow nobody. With Staff/Admin monitoring exceptions. But I see no reason they'd be looking at your CS collection any ways without having some sort of tip that it was violating some sort of site policy).
A personal thread somewhere?
This would work similar to a blog except you can set it up to have multiple full posts following each other on the same page. Instead of only viewing a single post at a time, and or a whole list of 'peeks' into posts. However, unlike the blog you've got no control over privacy, and someone replying would fill in space between your entries. So you'd either have to reserve several posts beforehand, or request no one to comment and if someone then does any ways have Staff remove it.
It's doable, personally speaking I've never saved anything there myself. But that's mainly because when I first tried it just seemed a bit confusing, and seeing how when I have looked at it there's only a couple of pages of content for the entire site? And a lot of this content is from a couple of members posting a ton of characters? I imagine a lot of others share my position here.
This would have great potential to organize it in a very big and elaborate manner if you just had a mountain of content to save and organize.
However, Iwaku's group system is currently outdated to the point they no longer allow new groups to be made. This means if you were to do this now you'd have be taking over someone else's group, and since the lack of ability to make new ones now has currently made groups a prized commodity? I don't see anyone letting go of theirs for something like that.
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So considering all those pro's and con's? If I ever do feel the need to save something specific I honestly end up using Blogs.
Reason for that isn't privacy though, but rather I just find it more convenient to just click on blogs to find it, rather than dig through the mountain of threads I've created to locate it.
+Threads can get closed, which would make it a hassle to update if I always had to ask staff to open it again. And by the time they do I might lose the idea I had.
But generally I only even use the Blog if it's just a basic idea overall and not for a specific RP (or group of RPs) in question. Because usually the respective RP's OOC's serve enough of a job as it is organizing the stuff and I don't often have a need to reuse characters in different RP's all together. Plus, it's not as if there's any employer I need to impress with a character sheet portfolio or anything.