I decided that I was not going to put up with a 60 GB limitation on my most used home entertainment appliance any longer. I didn't use a guide, but I enjoy doing things like that by myself. If you need a guide GamersReports has one. I got myself a 320GB 2.5" SATA drive and went to work. I used the PS3's Backup Utility to backup all the information on the hard drive to a third location for the time being. This saves movies, gamesaves and any PSN titles you have bought. Then you open the latch on the left side (or bottom if you stand up the unit) and take care of some screws. This was the hardest part of the whole ordeal. The screws in my HD were so tightly wound that I ended up destroying them in the process of taking them out. 
Once the HD is replaced, it asks to format, and boots. This really doesn't take long at all, afterwards you should go back into the Backup Utility and restore your saved data. This took an hour and a half for me (45GB of data). It is worth noting, I have a PSN account (morouxshi) and I keep my trophies synchronized with it. You do this by viewing your trophies in the Game area of the XMB. The backup process does not save your trophies, PSN does. The second the restore was done, I played some Pixel Junk Eden, then went to see my trophies, and during the sync they were downloaded back into the system. All in all, be careful that you dont strip your screws and have the right tools. Unlike I did. 
Once the HD is replaced, it asks to format, and boots. This really doesn't take long at all, afterwards you should go back into the Backup Utility and restore your saved data. This took an hour and a half for me (45GB of data). It is worth noting, I have a PSN account (morouxshi) and I keep my trophies synchronized with it. You do this by viewing your trophies in the Game area of the XMB. The backup process does not save your trophies, PSN does. The second the restore was done, I played some Pixel Junk Eden, then went to see my trophies, and during the sync they were downloaded back into the system. All in all, be careful that you dont strip your screws and have the right tools. Unlike I did. 