It seems to be common consensus that water that comes out of the tap is "clean" and that it can be added directly to your fish tank for a water change.

In my experience this is furthest from the truth.

You need to pre-condition your water before adding it to your tanks.

Tap water contains many chemicals harmful to your fish, chlorine and chloramines are just a few. They are added in the final stages of filtration by your local municipal water company to abate the growth of bacteria and other organisms as the water sits in holding tanks and makes it trip back to your house. Each system is different and each one adds different things to the water based on the composition of your pipes and the bio-load of your local population.

Moreover, municipal water is seldom static and they are always adjusting the amount and type of chemicals added.

How do you go about pre-conditioning and pre-filtering your water?

There are a few ways to go about it. Following are two methods: The first is the simple, infrastructure free method which provides a minimal amount of pre-conditioning, and the second is a bit more involved but provided high quality pre-conditioned water for your fish

1. Simple Pre-conditioning
   For this method you'll need a couple of 5 gallon buckets, or other containers that hold water, some Prime (or similar dechlorinator), and whatever salts or buffers that you need for proper water chemistry.
   1. Fill up your buckets/containers with the required amount of water, and try to get it as close to the tank temperature as possible
   2. Add the appropriate amount of dechlorinator
   3. Add any salts or buffers, as needed
   4. Let sit for about 20 minutes and check the water parameters to make sure they are as close to the existing water in your tanks.
2. High Quality Water Pre-conditioning
   
   1. Aquire a food grade plastic barrel or drum from http://www.bayteccontainers.com/, there are other companies out there too, but these guys are great to deal with. Make sure that you get one with an open top and not just the two bung holes in it. You can also get drum dollys from these guys that make moving around a full barrel a snap.
      
   2. Get an extra heater, airpump, and airstone. Make sure that your heater is rated to keep 55 gallons up to temp. Set the temp so that its the same as your tanks.
   3. Get an AquaClear 200 or equivalent hang-on filter and fill it with carbon. You don't really need to focus that much on removing particulate matter, the key is filtering it through the carbon to remove any harmful chemicals.
   I have fabricated a custom setup that makes this very easy. As soon as I can get some more time, I'll post some photos and how tos. I'll also be selling them shortly, so check back soon and you can just order one.
   
   Each time you fill it up, rinse your carbon (change it at least once a month), and then add your heater, airstone and fire up your filter. Filter your water for at least 24 hours before adding it to your tanks and you can be confident that you'll be adding water that is safe for your fish. Moreover, this setup obviates the need for expensive dechlorinators and water conditioners.
   
   If you need to add any salts or buffers to increase the salinity or PH of your water this is the best way to do it. All of the additives will get a chance to dissolve and you'll be able to keep the water parameters as even as possible even when making water changes.