One of the best pieces of advice that I can give is to write every single day. I can't take credit for that advice - many, many authors have been saying the same thing for years. My experiences, both those times when I do write daily and those where I don't, have firmly convinced me that daily writing is essential to becoming a good writer.
I can see the questions and objections running through your mind. Why do I need to write every day? How can I find the time? What can I possibly find to write about? And what happens if I really, really need to skip a day? Here are my answers.
Why write every day? There is one and only one good way to get better at anything: practice. You need to practice writing (I know I sure as heck do). The more often you practice, and the more consistently you practice, the better you will be. Imagine a football team that only practices once a week. How good would they be? The same applies to writing. Without daily writing, you are not going to get better very quickly, if at all.
How can I find the time? The answer is you don't find the time, you make the time. You have to set priorities, and writing should be one of them. This does not mean that you let other important things go--we pretty much all have day jobs--but you do need to figure out where writing will fit into your life. If you can, set aside the same time every day to write. You do not need to spend hours writing every day, but try for at least 10-15 minutes.
What should I write about? The answer is: anything. And I really mean anything. You don't always have to be writing a story or poem. Write a diary entry. Practice writing descriptions by describing your friends or your house. Write a letter to your mother telling her what you have been up to lately (it's good writing practice, and she would love to hear from you). Write a guest post for a blog that could really use more content, but whose author can't always come up with good ideas (hint, hint). Write anything you want; just write.
And what happens if you have to skip a day? Skip a day. Yes, you should write every day if you can, but it won't be terrible if you have to miss one day. The important thing is, try not to skip more than one day a week, and never skip two days in a row. Once you start missing days, it is very easy to keep missing them. Pretty soon, you are not writing at all. So, skip a day if you have to, but not more than one.
Now, it's time to practice. Choose something to write about, and for the next ten minutes do nothing but write.
(No ideas coming to mind? Write a story that ends with the line: And that's how the cow ended up on the roof.)