I got the best strategy to practice IELTS writing task I from here:
http://www.testbig.com/comment/42566#comment-42566
it may be useful to other users:
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Step 1, Take about 10 minutes to read one essay by one of the following users:
http://testbig.com/users/dilbar
http://www.testbig.com/users/mrmo2610
http://www.testbig.com/users/xthangbm
or any other sample essays.
in this step, try to analyze how they develop essays, and even try to remember the words, phrases, and sentences they used in the essays.
Step 2, Write the essay in 20 minutes by your own. In this step, don't read the sample essay, but you can apply whatever you can remember from the sample essay to your essays, like words, phrases, and sentences. (It is not shameful at all)
Step 3, Submit the essay to testbig to check out. After the essay submission, you can read the sample essay again and compare. You will understand the differences between the sample essays and your essays.
Step 4, After received the feedback from testbig, double check the flaws of the essay like grammar, structure issues. So don't repeat those issues in the future essays.
Step 5, Try to apply the tips you learned from this sample essay to the next one.
Step 6, Continue the step 1 to step 5 and you will find out the big improvement.