I recently had the misfortune of losing the hard drive in my iPod.  It was about 1 year and two weeks old, so the warranty was expired when it first started giving me problems.  I ran some hard drive utilities, such as SpinRite, and got it back in good enough shape to work.  Then it got worse and worse, until the thing wouldn’t sync at all.  I even tried some other programs, in the hope that iTunes was the culprit.  It wasn’t, so I started doing research.
I bought a tool kit with instructions online.  The instructions came through email, so I had those that day, but the tools took 9 days to arrive.  The day after I ordered the tools, I found a hard drive for under $90 that had the recommended model #, and ordered it with ground shipping.  I received the drive in 7 days, but the tools weren’t here yet, so I sent the guy I purchased the tools from an email asking if they had been sent yet.
The next morning I was too excited to wait for the tools.  I knew that using a metal screwdriver would ruin the finish on the iPod, so I tried my thumbnail.  I got my nail between the front and back panels in the top left corner of the device, and used my other thumbnail between them too.  I slid my nail down the left side and across the top and made my way all the way around.  It just took a very little bit of pressure to separate the two panels.  You can’t yank too hard, though, because they’re connected with cables that could be damaged if yanked out.
It was relatively easy to see what to do.  If you look hard enough you can find the instructions on the Internet for free.  You only have to separate one cable to split the two panels apart.  When you lift up on the hard drive there is a ribbon connection.  That connector was tricky and it would have been damaged if I hadn’t read the instructions and seen a picture of how to loosen the connection.
If you have experience with taking things apart, the only thing the instructions add is that one connector.  Although, I really think I could have figured it out with a little bit of time.
If you have a small plastic screwdriver or if your thumbnail is long enough and strong enough, the tools are a waste of time as well.  Mine were just about at the length when I usually clip them.  My nail got a little scraped from the separation, but I clipped them shortly after I changed out the hard drive and verified that it worked.
I should have taken pictures, but if anyone asks, I’ll take it apart again just to get the shots.  I received the tools in the mail later that day, so they’re in my cabinet if anyone has weak or short nails.
The one thing I looked for when looking for a hard drive, was to only purchase one that was advertised as new.  I would be wary of any that are refurbished, as those would normally come from another iPod, and might have the same type of problem my iPod problems started with.