You've already got some great suggestions, but I thought I'd throw my two cents in!
1) Write for you! If you write because you enjoy it and not for the sake of getting reviews or making others happy then you will find yourself with the motivation to keep going! All too often we see a lot of writers who start off hoping for tons of reviews and then feel such disappointment that they cannot keep writing. Reviews and making others happy is just a perk!
2) I think it's great that you are actively asking what you can do to improve your writing. Don't stop asking questions. Ask other writers what they have done, ask for opinions and constructive criticism when you can get it.
3) Get a good beta. Your best friend may be a nice person but they might not be great at being a beta. A good beta will understand grammar, of course, and the basics of composing a good plot line - but what really sets the okay beta from the really good ones, in my opinion, are the ones who aren't afraid to look it up if they don't know. My beta does this all the time, even if she just has a hint of doubt, to assure that she is giving the most valuable information she can.
That aside, you also need someone who is compatible with you. If you frequently have ideas for plots that are less than mainstream (Like me, who writes a lot of horror and smut.) you will need a beta who is comfortable with those subjects. You'll also need a beta that has the ability to keep up with you, finding one who has a schedule that can work with yours is best for both of you.
4) On opinions from readers, fellow writers, and your beta: Don't take things too personally. You write for yourself, remember?
Take everything with a grain of salt and try to see the constructive way you can use it for your improvement. It will help you grow as a writer.
5) Write frequently. As frequently as you can. The more you write and work towards improving your writing the better you'll become. Even if you don't feel like writing (and some days you won't!) write a little something.
6) This is the most important thing... above all else...
have fun!