Be consistent. Press the finger to the scanner, hold it there for a moment, and scan at the same angle each time. For example, if you register your finger with your hand open, hold your hand the same way each time you scan.
Read the prompts carefully and ensure that you wait for the prompt before you swipe.
When scanning, press hard enough to fully contact the sensor.
Wipe dirt or debris off your finger before scanning it so nothing interferes with the scan. Rub your fingers together to stimulate the natural skin oils, making your fingerprint easier to detect.
If you are having trouble registering a specific finger, try to register a different finger.