Quality of the Beans
Yes, I said BEANS. It is important to fresh grind your coffee. If you are not doing that and really want a good cup of coffee - this would be the first step.
The second is to get freshly roasted coffee. Coffee that you find in a grocery store can be months up to a year old. Health food and gourmet stores are better and then, of course, there are local roasters - and purchasing online. Some roasters (like us at Java Planet) actually put the date roasted on every bag - not just an expiration date.
If you have been through the above and your coffee is over a month old suspect old/stale beans.
If this doesn't change the bitterness check out these other reasons for a sour cup: