In order to improve your writing skills : practise writing,writing,writing...you get the point

A good idea, already suggested, is to buy an essay-book and start writing on your own essays. Moreover, you could practise speaking english with your friends or join an english-speaking forum (be active, too!).
Concerning listening : Watch CNN/BBC/whatever at the television. Watch english movies without subtitles or, if the movie has subtitles, try looking at the center of the screen instead of the subtitles. Try to understand what the actors are saying, instead of reading the lines on the screen!
Last but no least, in order to enhance your reading abilities, as well as overall text comprehension, you should read english books,newspaper and the like. I can tell (from personal experience) that reading books has helped me the most. By the way, you can find newspaper articles on the web, no need to buy the actual paper
Hope i helped!
Edit : Another thing that has proved a major help (for me) in english, is visualizing stuff i hear, instead of trying to translate it!
Sorry for the long post in english, was too bored to Ctrl+Shift