Writing is like singing. Or winking. Or driving. Hypothetically, everyone is born with the ability to do it—but only some people are good at it. However, like all those other things, there are tricks to get better. Here are five of them: #) Read. Nothing gets you in the mood to write more effectively than reading other writers who wrote great things. Read voraciously—read everything. Read ads, read books (classics, not classics, fiction, non-fiction), read the back of cereal boxes. All the text you see was written by someone; some of it is good, some of it is awful. Learn to see the difference, so you can do better when it's your turn.
#) Write constantly. Nobody ever got better at anything without practicing. Writing is like a muscle. If you're not exercising it, it'll be flabby and weak, and all the kids at the playground will laugh when you fall off the monkey bars on the second one (Morbid. Maybe not true. But what if it is...).
#) Let other people read your writing. It's scary. It's daunting. It's opening yourself up to criticism, and worse: what if they don't like it? But if they don't like it for a good reason, then you can make it better. And once the scary part is over, you'll be a better writer. And indebted forever.
#) Write more. So maybe this is a dead horse I'm beating. But maybe, just maybe, it's the most critical aspect of getting better. It takes babies weeks to learn how to walk. They're not experts on the first try. It likely takes longer than that to be an excellent writer—but you never know until you start.
#) Follow the rules. The old adage, "Rules are made to be broken," is not true for learning how to write. Excellence comes from mastery. Mastery comes from practice within the guidelines. Once you're an expert, you can bend and tweak and twist the rules, because you know how they work and what they're there for. Until then, learn them. Practice them. Obey them.
Do these things and you'll become a better writer, maybe without even realizing it.
What other people have said.