When choosing service accounts, consider the principle of least privilege.
Don’t forget to backup System Databases (Master, Model and Msdb) while configuring Backups for your SQL Server Instance.
Always keep Data and Log files on separate Physical Disk.
Avoid using SELECT * FROM <Table_Name>, try to use only the required Columns & Rows – if possible.
Configure start-up mode of your cluster SQL Services to Manual.
Try not to place user data on Primary File-group.
If you are using Fill Factor don’t forget to enable Pad Index.
Change the recovery model of your databases to SIMPLE where you don’t need point in time recovery and don’t wish to configure Transaction log backups also (Testing Databases etc.)