1. It should be pure
Since coffee is mostly water, you should be conscious of the water that you're using. It shouldn't have a discernible taste to it, and it shouldn't be completely distilled. The mineral content in natural spring water actually has properties for proper extraction.
Avoid tap water, if possible.
2. Temperature is important
Many automatic drip coffee pots don't heat up enough, sending too-cool water through your coffee grinds. The result is usually a slight sour-tasting pot of coffee. There's not much you can do to fix this, except invest in a manual coffee brewer (from $10) or a automatic drip pot approved by the SCAA (Bonavita BV1800- $124).
If you use any sort of manual brewing process, you probably use a stove-top kettle to heat you water. Once it starts to whistle, turn your stove off and let it sit for one minute. That should get it to 200-205 degrees F, the perfect temperature for brewing coffee.
3. It's not just for brewing
When you're waiting for your coffee to brew, pour some of that 200 degree water into your cup and let it sit until you're finished making coffee. The result is a cup that stays warmer much longer.
If you're using a manual process (french press, Chemex, pourover), do the same before adding your grinds. Preheating the vessel will keep the coffee hot through the entire brewing process, which is great for proper extraction.