Using water that isn’t hot enough
Many commercial drip brewers never reach the ideal temperature (195 to 205 degrees) for perfect coffee. Water in this range extracts the most flavor from the beans. Switch to a French press or pour-over method (Chemex or a ceramic filter), where water just off a boil is poured over ground beans and steeped for three to four minutes. These generally cost around $30, which is cheaper than the average electric brewer.
Using beans that were ground a long time ago
Buying pre-ground beans is a no-no. The minute coffee beans are ground, they start to release their nuanced flavors. The longer you wait to brew, the more the flavor will have diminished. Grind whole beans in the grocery aisle or do it yourself at home: Invest in a low-cost blade grinder or spring for a burr grinder for an even finer grind. Power it up while the water is heating up. (For bonus points, amp up the flavor by adding spices to your beans.)