One of my favorites is under Keyboard Shortcuts
You will see a few defaults there.
The one I use the most is completing my email address without manually typing it every time. Use the + sign at the top right of the screen.
Under Phrase, type your complete email address.
Under Shortcut, type @g, for example. Now when I'm typing a text or completing a login, all I have to type is "@g" and my gmail address types out in full! If you have more than one email address (work, home, junk), just assign the other ones with a different letter after the @ sign!
Other examples are your mailing address, phone number, work, title, phrases like "talk to you later" (I just type ttyl and it types it all out).