For those of you who wasted your money to have the dealer undercoat your car, you may quit reading. For the rest of you who were smart and saved your money, this could be for you.
I just undercoated my rear wheel wells and it only took me 5 minutes or so. Simply go to the local Autozone or whathaveyou, and buy a can of pressurized undercoating. There are several brands, but they all work the same. Simply jack up the car, remove rear wheels, and start spraying. It will help if you clean the wheel well before hand so the undercoating will stick better.
For those of you that are lazy... like me, you don't even have to remove the wheel. Just spray carefully and keep some towels close to clean up any overspray. My overspray was very minimal. Also, WEAR SAFTEY GOGGLES!!!! I can't stress this enough... you do NOT want this stuff in your eyes. I used to work at a dealership, so I know that it takes them less than 30 minutes to do a car(thus the waste of money), and I've seen a dood get this stuff in his eye... bad news.
You can also jack the car up and spray underneath... I will be doing this next week when I have access to a floor jack. Do yourself a favor and tape up any connections or bolts or anything you do not want undercoated. Saves a headache or two down the road.
GL! :D

next your gonna tell me that the "paint sealant" is a rip also. lol
very true on the undercoating. very true.

Paint sealant, fabric guard, undercoating... extended warranty... all a waste of money in my opinion. It is how the finance guys make money. You're better off to decline it all. You want paint sealant? It is called 'clear coat'. NOTHING is going to save the crappy paint we have on our cars. Your best bet is to wash it at least weekly and wax it every couple of months. Make sure that you use bug/tar cleaner to remove bugs from the front and keep an eye on your rock chips. Be sure to touch them up when ever you see them. Try not to rub up against your paint... a belt buckle, or something might take a nice size chunk out.

Oh well. :)