Create a directory on your computer for each website project.  Later, you will have a directory for the different themes and plugins you buy, but that will come later.  Make a Word or Excel document for each project and start taking notes on what you are doing and what is working or not.  As you progress with your website, you will have layers upon layers of plugins and changes made.  Keep a note of these and whatever other services you have integrated.  This will help a LOT when you need to upgrade or something goes wrong.

Link: None.

Cost: None.

Time: One second right now and will save you lots of time and headache later!