* Record your own voice and listen for pronunciation mistakes.
Many people hate to hear the sound of their voice and avoid listening to themselves speak. However, this is a very important exercise because doing it will help you become conscious of the mistakes you are making.
* Be patient.
You can change the way you speak but it won't happen overnight. People often expect instant results and give up too soon. You can change the way you sound if you are willing to put some effort into it.
The following are some book references to help you improve your oral English:
- Spoken English by R K Bansal and J B Harrison
- Pronounce It Perfectly In English by Jean Yates, Barrons Eductional
- English Pronunciation For International Students by Paulett Wainless
Dale, Lillian Poms.
As for your written English you need to practice a lot of reading and writing.
Lots of chatting, exercising, listening and speaking will help you use the words efficiently and with lesser mistakes in structure ,grammar and pronunciation,which will help improve your writing skills.
You could possibly join some workshops for improving writing skills.
It is a gradual process and if you practiced on a continuous basis with real dedication you will sense a tangible progress and improvement sooner or later depending on your efforts.
Good luck :)