The ratio problem
The most common reason drip coffee tastes weak or bland is the wrong coffee-to-water ratio. Many machines and pre-packaged coffee pods are calibrated to a 1:18 ratio (55 g coffee per litre of water). Specialty coffee standards recommend 1:15 to 1:16 (60–66 g per litre).
The simplest improvement you can make to drip coffee is to add more coffee. If you're using the scoop that came with your machine, try one and a half scoops instead of one. If you weigh your coffee, target 60 g per litre of water.
Water temperature matters more than you think
Coffee extracts best between 91 and 96 °C (195–205 °F). Many inexpensive drip machines heat water to only 80–85 °C, which isn't hot enough to properly extract the sugars and aromatic compounds that make coffee taste full and sweet. The result is coffee that tastes sour, thin, or just plain flat despite a correct ratio.
If you can't control your machine's temperature, a simple test: brew a cup and if it tastes noticeably sour despite using enough coffee, temperature is likely the culprit. Look for a machine with a SCAA-certified brewing temperature, or pre-wet your grounds manually with a small amount of boiling water before brewing.
Grind fresh
Ground coffee starts losing aromatic compounds within 15–30 minutes of grinding. Pre-ground coffee in a bag has already lost most of its complexity by the time you open it. Even a basic blade grinder used right before brewing produces noticeably better coffee than pre-ground beans stored for weeks.
For a drip machine, use a medium grind — like beach sand. Too fine and the coffee is bitter and potentially plugs the filter; too coarse and it's weak and under-extracted.
Clean your machine regularly
Mineral scale from hard water accumulates on the heating element and affects both temperature and flow rate. Descale every 1–3 months depending on your water hardness. Run a cycle with equal parts white vinegar and water, followed by two cycles of plain water. You'll immediately notice the coffee brews hotter and faster.
Also rinse the carafe and basket after every use — old coffee oils left to oxidise will make your next brew taste rancid and bitter even if everything else is perfect.
Use filtered water
Tap water with high chlorine or mineral content noticeably affects coffee flavour. A simple Brita-style filter removes most of the compounds that interfere with coffee taste. Avoid using completely demineralised water — it actually extracts less efficiently than slightly mineralised water.