Not to be confused with being materialistic, materialism is defined to be the doctrine that matter is the only reality, and that everything in the universe, including thought, will, and feeling, can be explained only in terms of the physical. A natural question which arises from this is how exactly to deal with the existence of one's consciousness.
A recent Time article by Steven Pinker discusses this topic and remarks on how scientists are systematically uncovering new secrets regarding the human brain. It's a fascinating read and makes a number of interesting points. In summary, neuroscientists have reason to believe that every aspect of consciousness can be tied to the (physical) brain.
Consider, if you will, the Star Trek transporter beam. Let us assume hypothetically that we will eventually develop this technology. The transporter beam works by reading your molecular makeup, dematerializing your body into "energy", and finally rematerializing your body at some other location. This whole process presumably occurs close to the speed of light and is thus a very efficient method of transportation.
The problem with this technology is that it seems to naturally allow for the ability to duplicate humans (this was used as the plot device to motivate the Star Trek TNG episode Second Chances) by simply failing to dematerialize the original. In this case, there now exists two copies of the same person. Since these two people are indistinguishable from each other and carry with them the exact same past experiences, they will both believe they were the original person before using transporter beam.
The problem runs even deeper once you consider the original intended use of the transporter. The transporter is supposed to dematerialize the original copy of the person. That is to say, the transporter beam creates an identical copy of a person somewhere, and then kills the original.
So there's the dilemma. When you get transported, you die and an exact copy of you gets created somewhere else. Functionally, it appears as if you were indeed transported from one place to another, so it doesn't matter to society which copy lives and which dies. But the fact remains that you yourself don't actually get transported, but are replaced with a new person who is identical to you in every way. This of course leads to a number questions regarding the definition of life and self and so forth, which was the entire point of considering this scenario in the first place.
Materialism suggests that your consciousness is simply the result of chemical and electrical activity within your physical brain. The essence of your being is a blueprint that, given the proper technology, is easily copyable, just like any piece of software. If we find this view plausible, then we must reject the notion of having what many call a soul. Taking this another step further, we must also reject the existence of an afterlife.
Though this seems to be a particularly radical line of reasoning, it actually has very little impact on your everyday life. In fact, as soon as you accept this argument of materialism, you can then proceed normally as if this argument didn't exist. I'm also of the opinion that free will is an illusion (i.e. not due to some fundamental notion randomness within our universe). But again, as soon as you accept this, you can effectively ignore it almost completely. Functionally, everyone still reduces to the same kind of selfish agents they were before accepting this argument.
The biggest change required is to reject any religious notions of an afterlife. This, I think, is a good thing. As Steven Pinker so aptly pointed out, believing in an afterlife necessarily devalues this physical one, and "nothing gives life more purpose than the realization that every moment of consciousness is a precious and fragile gift."
In the long run, of course, I want to live forever.