Read often. The more you read the more you are exposed to vocabulary, different types of sentence structures and punctuation patterns that will help you if you have internalized them when you are writing original material in English. Don't just read, but make sure you understand what you read. Join a book club or meet for book discussion with a native speaker who can help you check your understanding and check your grammar.
Write in a journal or blog. The more you write, the more opportunities you will have to practice your developing English writing skills. According to the LEO Network, it's better to study for a short amount of time each day than for large blocks of time on only a few days each week. For this reason, make sure you write a little bit each day. When you feel comfortable sharing an entry, do so with another native English speaker and discuss with that speaker the effectiveness of your writing. Make a checklist of your most common mistakes and use that list to guide your study path. You can also use this list to help you edit formal assignments, letters and other communications.
Spend time studying and practicing the specific grammar rules with which you are struggling with. If you sit down with a grammar book and tell yourself you are going to learn English, you probably won't get very far. However, you should choose at least one item on the list you created above to practice for a few minutes each day. Carefully read over the grammar rule associated with that item and do a few of the exercises associated with that item in your grammar book.
Ask others to proofread your work. Whether you're writing e-mails or a dissertation in English, having others proofread your work can be a wonderful learning experience. Complete the item to the best of your ability, then sit with a native English speaker as she reads it. Ask her to be as specific as possible about areas that are confusing or grammatically incorrect and mark them on your paper. Next, write another draft of the communication in which you attempt to correct the errors or confusion. Ask your native speaker to proofread the paper again. Do this until your work is error-free.