Upgrading the HD in my beloved Macbook was surprisingly simple:
Step 1 - Buy a new drive and prepare it with Disk Utility: 'Erase' the new disk and let it format as the default (Mac OSX Extended Journaled)
Step 2 - Download some cloning software. I used Carbon Copy Cloner (donationware).
Step 3 - Clone that bad boy. CCC lets you do this whilst the drive was active so there's no need to reboot into a live CD.
Step 4 - Give it some time . . . about a minute per MB. It took a little over 3 hours to clone the 200 GB or so I had, so being a little drunk by this point I went to bed and left it going.
Step 5 - Optional: blessed sleep followed by mild hangover.
Step 6 - Test it's ok by rebooting and holding the 'option' key so I can boot from the drive in the caddy.
Step 7 - Swap out the drives.
Step 8 - Compulsory: bask in the glory of just how 'on' your geek was.
After you're done:
If you used the defaults, your drive is probably now called 'untitled' instead of 'Macintosh HD'. If you want to change the name, you can do it via 'Get Info' in the context menu. I called mine Rosebud.
You have the wit to find alternative instructions if this didn't tell you all you needed to know.
You have some kind of USB or SATA caddy you can use to connect the new drive to.
You can find Disk Utility.
You have the wit to find out how to remove and refit a drive.