1. Try to write in English
Why dont you try to write in English a simple sentence or shortmail such as your mail. I'm sure you can do this.
2. Just read anything in English
Newspaper, story, magazine or anything that you like. Learn how the writer arranged the sentence and paragraph. I'm sure you will get something. New vocabularies, how toarrange a good sentence etc etc
3. Prepare your dictionary and grammar book
When you write,always check and re-check again that you use the correct word. Also,check the grammar too. I always use word processor program (such as,MS Office or Open Office) to help me check the spelling and grammar.Trust me, it's much easier than doing it manually.
4. Try writing blog in English
If you like to write, why dont you try writing blog like me. You can write in free blogging software such us Blogger, Wordpress, Multiply etc. Or,microblogging such as Plurk, Twitter,Kronologer. Some of my friends did this. They just wrote their daily/hourly activity in short message like SMS. If i'm not mistaken a message should be not larger than 140 characters. Hmm...easy, isn't it? As far as I know, indonesianmicroblogger is just a few. So, they had to interract with other user who are foreigner and speak the same language, English. They said that their skill has improved than before.
5. Always learn and learn
Last but not least, keep the spirit and your confidence. If you are not trust yourself, then who will? So, keep practice and practice.