I'm sure you know about the 'Cambridge series' online practice tests available....
if you haven't come across already try this
http://ieltsonlinetests.com/catalogue
there are about 60practice tests for all 4 modules....practice all of them...(its boring after first few trials...

...esp listening to the instructions each time you do the listening practice test...
for speaking practice talking on each topic and record it using the microphone on ur mobile headset...listen urself after each and try to improve on duration and grammatical correctness and style...
it will help u when a random topic is thrown at the real test...
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writing needs practice...for speed and to write in organized fashion...
task 2 carries 6points out of 9 ...so make sure u write in a structured way...
have an introduction paragraph/then the pros and cons or for and against arguments on the topic / conclude with your opinion after considering all the pros and cons...
it is impossible to write all 60 practice topics...however try to write down the key points you will fill the essay with ...so it will help you quickly fill u with ideas at the real test...no topic will surprise u much and you will be able to think and write in an organized manner...
All the best....