Assuming I have the space I like to keep the boxes for my electronics purchases. This has come in handy for insurance claims, warranty returns, selling the items, and of course for finding the manuals.

I have ziplock bags for my extra cables which I label not only based on the type of connections they posses, but also what device I got them with. For example one of my cables is a simple USB type A to B cable, but my label tells me I got it with a USB 1.1 device and so I know I may not get full USB 2 speeds when using it with newer devices.
The bags are also sorted based on the general type of cable such as audio cables, usb cables, power cables, etc.

I also have a folder on my computer where I keep read me files and such so that I don't need to go digging them up on the web. This not only saves me the time of having to find the file online, but has saved me in the past when the product was discontinued and the file was pulled from the web. Another huge bonus is that as a locally stored file it is included in the index of my computer's search feature.

I have this problem in spades - but hit on a very effective solution some years ago. I use the 1 gallon size Ziploc Freezer bags. Whenever I purchase a gizmo/device, I put the receipt and all the stuff from the box (documents, extra cables, chargers, etc.) in the Ziploc bag. These all go in a plastic filing box (actually, several of them by now).

You can write the item name and date purchased on the outside of the bag with a Sharpie marker to make it easier to identify later.

I don't like to mix all the cables and manuals together - I'd never be able to find them. This way, if something goes wrong with your item, you have everything in one spot, including your purchase receipt, so you can return it.

I have this problem too, I have a bit of a mixed answer for you

If your item is:

Going to be Sold in the future: keep the box and everything that came with it, in the box and store the boxes in a closet under a bed or somewhere you can easily find it when needed.

If your keeping it till it looses its usefulness: Ditch the packaging if you don't have space for it. Consolidate the stuff, if you get alot of the same items like usb cables they deserve their own home. If its something small that can easily be digitized, like a manual or a cdrom, scan it or rip a copy of the disc and store it neatly on your PC. If its something bigger like a thick manual I suggest the filing cabinet as mentioned in an earlier post. If you don't have a filing cabinet you can pickup a bankers box from staples or your local office store. Instant filing cabinet, be sure to label your folders neatly.

Lastly I would start by finding some software to help you remember what you have. If your like me and you have way too much stuff. You need a home inventory system. I have been looking for something that is an efficient way to enter items and store useful information about them. I have yet to find one that really does a good job at this.

I have tried using microsoft money's built in inventory system, it works fairly well for electronic stuff that comes with warranties, because you can enter in all that information (length of warranty, purchase date, purchase cost, current value, manufacturers website, phone number, and miscellaneous field) you can also categorize the items by room if your super organized. This may be more time consuming than necessary, but until i find a better system this is what i use.

So I use a combination of the above techniques to keep everything organized. It is time consuming, but whatcha gonna do.

I really wish someone could create a nice home inventory system. Something that could be combined with like home automation where you could access the information in several rooms. I would love to have something that worked with a refrigerator/pantry so you know when something is gonna expire/your running low on it. Might be a little over the top, but i feel that with the right implementation it could solve a lot of peoples problems.

Perhaps you could buy a file cabinet and have some folders labeled with the names of the items that you purchase. In the folders you could put the manuals and extra cables and extra parts. Another alternative would be to purchase a set of clear plastic drawers that you can label with the names of the items that you have purchased. The drawers would be easier to store the extra cables, and power adapters.

You can scan all of you papers, that way you can store them in folders on your computer, nicely labeled. As for the manuals, since you probably don't want to scan many pages, you can put them in shoe boxes along with the cables, and don't forget to label your shoe boxes.

Or you can always use a 3 ring binder with sheet protectors for your manuals warranties etc...

These days, practically all documentation that comes with electronics is available online (usually at the manufacturer's website, otherwise a bit of Googling will generally do the trick). I find that it makes things significantly easier to find and download said manuals as soon as the item arrives, and keep them in a separate folder in My Documents ("My Manuals", of course), naming the .pdf's with the make and model so that I can reference it easily. Also, you can search a .pdf the same way you can search a Word document, which makes it a lot easier to find something in it that I'm looking for than it would be to leaf through it, especially if it's a big one.

Accessibility, ease of use, no clutter...what more can you ask for? Keeping digital copies of documentation is the perfect way to 'archive' manuals and stuff.

Download them all from the websites of the manufacturers, file the warranties but write the expiration date on the front real big so you don't keep it when it's useless. Cables can be tied, bound, etc and put into a box and hidden away, but keep one spot for cables if you do this. If you have 20 places of hidden cables you won't find anything. They don't need to be air conditioned so a garage or attic works.

I like mckoss answer about the plastic bag. I would also tape the plastic bag to the side of the item if I really had trouble keeping things together. We have an Okidata color laser printer that comes with a clear plastic pocket on its side for manuals.

Personally I don't do that. I keep everything, and then file them in hanging folders separating receipts from manuals. I stapled the sides of the folders for the receipts and manuals so the smaller ones don't fall out.

As for wires, I bought a hanging shoe organizer and made labels for the different kinds of cables, and then stuffed them in the shoe holes. I also use a cassette organizer for my adapter/charger collection. It has 9 holes each designed to accept say 6 cassettes, so I have one hole for 3 volt adapters, one for 4.5 volt, one for 6 volt etc. Very useful.

You should really take a trip to a The Container Store to get ideas.
http://images.containerstore.com/images/home/elfaSale2008/people.jpg

I store them all in a file cabinet inside of File Pockets or File Jackets. Then name the file whatever the product is and the date it was purchased. For example: "Dell Inspiron 1525 (8/08)" Then place all my paperwork, CDs and manuals in there.

File Pockets:
http://www.staples.com/office/supplies/p4_Staples-Expanding-File-Drawer-Pockets_153167_Business_Supplies_0_10051_SC1:CG15:CL140619

File Jackets:
http://www.staples.com/office/supplies/p17_Staples-span-style-color-green-100-Recycled-span-File-Jackets-with-Reinforced-Tab_11870_0_Business_Supplies_10051_true_FEATURED:SC1:CG15:DP2248