Tip #9: Whatch out for syntax candy Eg: foreach loop.
This is one of the most repeated tips for CPU optimization in Unity. Because of the old version of the .NET framework that Unity uses many things are not as efficient as they should be. Since this is not changing soon you should get used to replace "foreach" loops for "for" loops and "lists" for "arrays" once in a while ;) More information on official code optimization tips here. General good code practices from any OOP language apply here aswell, be sure not to miss them :)