Polyurethane bushings are an easy and effective way to upgrade your car suspension. 

The stock bushings are soft and squishy, and become less effective the more you drive your car. You may have failed bushings right now, and not even know it. 
Installing rear sway-bar bushings is not only easy, but it is also very cheap. At $10 for parts, and 20min for installation, there is no reason not to do this mod! While bushings will not make as dramatic a difference as a stiffer sway bar, they do make a very noticeable difference for a very cheap price tag.

 
Here is what comes in the package, two bushings and some extra tacky grease. 

Start by lifting the rear end of your car. If your car has not been lowered, you should be able to fit under the back with out lifting it at all. I was able to with ease. 

These are the two spots, where the bushings will go in. As you can see, each bracket is held in place by two 12mm bolts. Start with one side, and remove each bolt. The bolts are on tight, but you shouldn't need a breaker-bar or penetrating grease. Once the bolts have been remove, you will be able to pull the bracket itself off with little effort. From there, pull the old bushing itself out as well. The bushing has a break in the back, so pull it straightforward and it will pop off.
  

* Once you have the bushing off, take a look at the old one. Shown here, mine was disfigured, squished, and cracked. 
  * 

Open up the package of grease, and using a q-tip spread the grease all around the inside canal of the new bushing. Once the bushing has been greased, pop it onto the sway bar. The brake in the bushing is in a different location than the old one, so you may have to slightly pry it open to get it on the bar. Snap the bracket back on top of the new bushing, and bolt it back on. 

Repeat for the other side, and you are done!