What are your Favorite SQL Server Performance tips?

As a new SQL Server DBA, I am learning that it is key to understand how to enhance the performance of your SQL Server(s) as well as monitor to see if all is well. However, it seems from what I’ve read, this is more of an art than a science. I would like to ask what you have found as your bread and butter secrets you implement in monitoring and improving the performance on your SQL Server(s).

Gary