Reduce background noise as much as possible. Turn off your radio, turn down the fan on your heater or air conditioner, keep your windows closed, and ask any passengers to be quiet when talking to the app.
Speak louder. You may need to talk louder than normal for your phone's microphone to pick up your speech.
Enable or disable the use of Bluetooth for speech recognition. By default the app will use your phone's microphone for speech recognition even if a Bluetooth device is connected. In some cases, using a Bluetooth device may provide better accuracy. You can adjust whether or not Bluetooth is used for speech recognition in the "Speech Recognition (Advanced)" area of the settings. It may be necessary to experiment to see whether Bluetooth helps or hurts your accuracy. Note that many vehicle Bluetooth systems can only function as a speaker and don't support speech recognition. If speech recognition over Bluetooth is enabled, you can check to see if it is in use by tapping on the Bluetooth icon on the main menu screen.
If using your phone's microphone, try to position your phone as close as possible to your mouth. Use a car mount, cup holder, or shelf if possible.
Try adjusting the speech recognizer that is used. In the Speech Recognition (Advanced) section of the settings, try enabling or disabling the use of Google recognition.