Apart from all the extra reading and watching english sitcoms, read the endorsed CIE AS English book very well.
Make notes with a detailed list of examples. (Examples of Tones, Purpose, Genres, Audience etc.)
The most important thing in the commentary paper is to give an effect created on the reader.
Try to make a list for the various figures of speech and the effect created by 'em. Ofcourse, these may vary from the actual purpose of the figure of speech in few essays, but a general list should help you. Be sure to memorize them.
Examples
The writer uses long sentences ---Effect---> Wants to linger on the same idea/wants the audience to be fully absorbed.
Short sentences ---Effect--> The staccato effect arouses reader's curiosity. The aim is to hit home/jolt the reader into paying attention.
Use of repetition --Effect--> To build up interest/to make the sentence sound poetic/to attract the reader's attention.
Rhetorical Question ---Effect--> It's simply a reshaped statement to give it emphasis.
Many of these are from the Book itself. As I told you earlier, it's the best guide you can follow.
Try to make notes of your own (similar to the above if you want). Read the numerous examples of commentaries given in the book
and pick out the effects written.
Be sure to include the writer's genre, the tone of the writer, the audience and the overview of the passage in the first paragraph. That should fetch you some marks instantly. Also, pay close attention to the punctuations used, you should find something on punctuations and its effects in the book too.
Here are some more examples of effects created by different figures of speech.
Contrast - To create a mixture of different attitudes in the reader's mind/ To puzzle the audience
Hyperbole - To draw attention and amuse/shock/impress the reader
Pun - Again, to amuse the reader/create a light atmosphere.
Colon (Punctuation) - sets up an expectation
[A common effect for all can be 'to sound poetic/attractive']
Good luck, hope you do extremely well this time!