Study every day, even if it's only for a few minutes. Make sure that you're using materials that keep you interested and hold your attention - if you're not interested in the material, you'll get bored and quit. 

If you only need English for school and aren't personally interested in learning it, then find a tutor on italki that can help you on those specific topics you need. 

Hope this helps a little! 

There is nothing more than practice to polish your English and there is no shortcut to take in my view.

Good luck and if you want to practice with our international English conference online. please check my profile and add me in your skype contact.