Read regularly. By reading you not only have the opportunity to enjoy the beauty of the English language but also become more familiar with the different types of sentence structures that can be employed. Read an assortment of written works, including novels, newspapers and magazines. If still working to develop your pronunciation skills, consider reading orally.
Ask those around you to correct you. While correction can be hard to take correction, you will likely not learn without it. Ask your native English-speaking friends to point out errors they notice. When they do point these errors out, take the correction well instead of becoming defensive.
Review written works with an individual skilled in writing in English. Compose a draft of a written work as well as you can, then improve upon it with assistance. Instead of simply passing the draft onto another individual and asking him to look at it, sit down with him and read over it with him, asking him to tell you what to change and then make the changes yourself. Doing this will help you internalize the corrections.
Compose a list of common mistakes. Place this list in the front of your binder or planner. Add to the list as you notice errors. A great way to add items to this list is to review class assignments that are handed back and include things that your teacher commented upon.
Focus on one mistake at a time until you have explored all of the listed mistakes. Dedicate approximately a week to each mistake and try your hardest during this period of time not to make the mistake either in speech or in writing. By doing this, you can make it easier to correct the mistake than it would be if you were attempting to focus on a whole collection of mistakes at the same time.