To establish a clear process to keep track of leave requests, it is important to only use one communication channel: bulletin board, email, voicemail, SMS, etc. This centralizes all requests  in one convenient location, helping to prevent omissions.
Scheduling software are often used to keep track of leave requests. Such software allows employees to request days off or vacation. Managers can then approve or decline these requests using the software, and the employees are sent notifications to confirm that their request has been processed. Approved requests are automatically displayed in the schedule to minimize errors.