Knowing the rules doesn’t mean they have to be followed stringently. Most dialogue doesn’t follow Ze Proper English, and shouldn’t have to, unless you’re writing a book called How to Speak Uncomfortably to Others. Come to think of it, I’ve read books that should have carried that title.
You can use slang if you want to get jiggy. (And okay, now I’m laughing pretty hard, because that is SO not what I would say and I suddenly feel 100 years old while typing it.) Prepositions are sometimes all right to end sentences with. And you can begin a sentence with a conjunction. You can apply the oft-misused Random Capitalization. If you wish, use clichés until the cows come home. Even . . . yep, incomplete sentences. You can do all these things, as long as you are using them for a particular reason, and you are aware that you’re breaking rules.
It’s the unsuspecting people who need to be careful. The phrase, “Those who do not remember the past are condemned to repeat it,” attributed to George Santayana (and repeated in various forms by Edmund Burke, Winston Churchill, and others), can be changed to the literary version: “Those who do not know the rules are bound to break them.”
It all goes in order: learn them well, then break them sparingly. You don’t want to be breaking so many rules that your readers begin to wonder if you ever learned any to begin with.