Forgetting to shutdown an instance is a common cause of extra charges, yet it is possible to avoid that by settings alarms, reminders or auto-shutdowns.
Use the CloudWatch service to set alarms. For more information, refer to Amazon CloudWatch.
It is possible to stop the instances on schedule. Please refer to Save money by stopping and starting Compute Engine instances on schedule.
It is possible to set alarms via Cloud Monitoring. For details and instructions, please refer to Cloud Monitoring - Manage alarms.
Use the auto-shutdown feature: in the Virtual Machine details, go to Operations > Auto-shutdown.