Well, for me at least, it took years of practice and learning the hard way. I don't really have too much of my old downloads anymore, as when I first starting playing TS2, 3 desktops ago, I knew nothing about proper backup techniques and just assumed my computer would tell me if it couldn't fit something on a disc. What I was left with was several incomplete backups, so I was usually forced to completely redownload everything when I needed a fresh start/had to reinstall. Anything that I couldn't find again, I just let go. There are a couple of things I still miss, lost because the original download was lost too, (like this teen patterned bathing suit set from here; it was lost in a crash and no one has ever reuploaded it ) but for the most part, I got all my essentials back. Or my tastes changed. (Man, was it hard to let some of those xmsims hairs go. )

Anyway, I learned to be disciplined. I always had subfolders so I never really had a situation where I couldn't tell a hack from an object or a piece of clothing, etc. and I rarely downloaded from the exchange or downloaded lots, so I didn't have too many crazy file names going on. I definitely had to learn the hard way about putting pictures in every folder. I've cleaned out my folder several times and spent hours searching the internet for blog links or dead sites or meshes and can go on for at least another two paragraphs about hunting through all of Garden of Shadow's threads to figure out where I got something and who made it.

So now that me and the game is older and the potential for good cc to be lost because simmers aren't around anymore is greater, I don't let myself procrastinate when it comes to staying organized. The very least I do is make sure every item has a picture and every hack has a description and that I put it in the right folder. If a creator has uploaded several things together, I sort them, putting walls in my wall folder, male hair in its folder and so on. No more mega folders with different types of stuff. And I don't even close a webpage until I've opened the rar/zip and checked if it has photo. If there's just a button or a texture file/swatch, I make sure to save the bigger preview picture too. I put the creator name on every hack and I used to create txt files for them too, just copying and pasting the hack description, instructions, and conflicts, but now I'm switching to jpgs and I use print screen to take a picture then paste it in Paint. If I'm feeling really lazy, I'll bookmark the page for later. I wait to download until I can properly organize.

I get that it's not easy. It took years for me to make sure I had pictures of everything, mostly because I'd start and stop cleaning my folder. I'm definitely the crazy simmer who spent her time googling downloads and scrolling through pages and pages images and pinterest posts and tumblr links to find the right preview pictures and figure out where I got something--which is why nowadays I usually don't download something if there's no preview picture. It's definitely not fun loading up TS2 window mode and previewing objects and clothes and taking pictures if there aren't any, but I've done that too. So don't lose hope; it will just take a while.

I don't think I have it under control. My general downloadfolder is over 23 GB (mostly unzipped or unrarred files, so it's even more if I would have unpacked everything by now)
On a regular basis I do some sorting by putting the disorganized part into the right folders.
I of course also then run into 'rar or zip-names' that do not give me any inkling what it is. Usually when I open to take a look it's obvious what it is.
And partly I made the effort to indicate what's in it, but since I'm not really consistent when on a download spree a whole bunch is a surprise to me what's in. Especially the older downloads.
I sure try to put it in the right folder when downloading. When those are files that's a set I can't do that, since I tend to save things in a different manner and not by set or creator.
For example a rar that has a bookcase, some clutter, a light and so on. For me, those all go in different categories and then it becomes a thing of having to go one file at a time to put in the right folder. Pfff. Sometimes when I open a rar or zip and see it's a file like that I go on to the next file. Yes, that's lazy, taking the easy way out.

There are packages though that only contain numbers. I would have to open it in SimPE I guess to find out what it is.

The point to the story is that I can't keep up with the sorting and organizing versus with how much I've downloaded over time.
To me, it's an ongoing process and I will always be a step behind on everything. Oh, well.

Part of the way of I'm organizin and renamin things I took a page from Jessa's book then did it how I could understand it. I now put a letter in front of everything that represents what the different items are. For example if it's clothing it gets an A, if it's a build item an F, a mod is an X, and if it's a zip/ rar of mixed items it gets an I. So far I've used over half the alphabet in breakin things down in categories.

Does that mean you keep it as a zip or rar?
For me, I want to unpack things because if it contains various items I can't really put it in a category.
By unpacking the various items can go to their designated folders and subfolders.
My system's quite detailed. (599 folders)
That makes the organizing more work, but if I need to find something for in the game, that goes quickly.
At the moment I'm in the fase of going over what isn't categorized yet in some way.
State of affairs: 560 bestanden left to go, together 2,42 GB.

You can trick yourself by putting zip/rar files into the Downloads folder. Apparently, the game tries to read them, but can't, and can spend some loading time on this. I'm not sure if this is 100% verified, but it's worth to have in mind.

Things I haven't put into the game, I usually keep in zips outside of the game - unless I haven't used all the files, and/or there's some important info in the zip.

It might not be this but often if you download a house that comes with cc files, especially walls, Clean Installer changes it to numbers. I don't often grab lots with cc but if I do and there are walls I do them one at a time into the lots folder while manually renaming them to Wall_lotsname(plus number if needed) I really prefer when lots come with a separate link.

Yes, I keep the zip/ rar files. I have a folder on my desktop, for now at least since I'm sortin, but later will prob go into my docs, where all items I download for Sims2 goes. Those are what I'm sorting and renaming.