I'm shocked to know how many people have no idea that the keyboard shortcut functionality exists. Under General->Keyboard->Shortcuts
You can define word expansion shortcuts; for instance make a shortcut so when you type "pls" it will expand it to "please"... or "bc" expand to "because".
Besides that, you don't have to make a shortcut, you can just define custom dictionary words, say that it keeps correcting some word to "duck"... you can define the correct word, and it won't keep saying it's wrong... or maybe you always refer to your friend, typing "brian" and it keeps changing it to "brain". Just add an entry, and I'd put "brian" as the shortcut, then have it correct to "Brian"-- that fixes the auto-correct irritation, and makes sure it capitalizes his name, even if you don't type it that way. So in that, you can also use it to fix capitalization... for instance, normally (for me) "google" won't auto-capitalize. Not a big deal, but I can set the shortcut to be "google" and anytime I type "google" it will autocorrect to "Google".
Yet another smart use for this is if you regularly make a certain typo... make that typo a shortcut, then the expansion the correct one. Example? I have a shortcut to expand "ppl" to "people". For some reason I always freakin' type "opl". So I set up a shortcut to make "opl" also expand to "people". Problem solved! Now I can screw up all I want!
Finally use the expansion feature to its full capacity! I suggest "eml" to expand to your primary email... "addr1" to your home address, I have several domains for email, so I set up shortcuts for like @gm will become @gmail.com -- blah blah...
Another standard I use is I set up text shortcuts to become symbols that aren't in the keyboard, like arrows, symbols, etc. To make sure I don't accidentally make these appear, I use the naming convention where I start all symbols with an "s" in the shortcut. So for the TM (trademark symbol) I set the shortcut to "stm" and the copyright to "scopy"... I also have it when I type a double dash -- to change that to an m-dash (the character that appears when you hold down the normal dash key) and I have it set to change when I type "..." to replace it with the single ellipsis (hold down the period button). Seems useless until you start using Twitter, for instance... and an ellipsis goes from 3 characters to 1, and a long dash goes from 2 characters to 1. Then you start seeing the utility of these "smart" types of shortcuts.
Use your imagination and try to minimize what you've gotta type in your emails and texts! I type so much a day my fingers get freakin' sore... so I use these shortcuts and have this feature do the work for me... here's a list of several of them to get you started thinking:
"bc, nvm, ppl, w/e, w/, dl, ul, tmrw, srsly, smth, hh (haha), pls, ty, gn, prly, wu?, wyd?, bf, gf, esp" --- you figure out the rest, haha...
If anyone thinks of some more innovative ways to use this feature, please share... it's insanely useful for so many things.
... and it looks like Massie beat me to that tip, but I just expanded... on it.... (didn't intend that pun, LOL)