loopsngeorge
I can highly recommend going to the Essex section of the 11 plus exams site
www.elevenplusexams.co.uk/forum/11plus/viewforum.php?f=15
There's lots of DIY tips on there.
It's a real dilemma DIY'ing, isn't it? I did it with DD2 for the September 2012 exam. I'm not a teacher and have no expertise or training in the field of education. At this point in time, I don't know whether DD2 has "passed" or not until I get that magic email on 1st March telling us whether or not she has a place. So at the moment, I can't say whether I've DIY'd successfully, as all I have is her exam score. I can give you a few ideas for the CSSE exam, though 
The CSSE English comprehension is notoriously challenging. Starting this early, means you can hopefully encourage your son to write in full sentences and develop his inference skills - absolutely imperative. It's also very important for him to be a competent speller - make sure he knows the main rules and the "bogey" words that often catch children out
. Good punctuation and to a lesser degree grammar, are also vitally important. In the past, there have been two sections which test spelling and punctuation - on average, these sections account for around 20 of the total 50 marks. Confident children can easily pick up a large number of marks by successfully completing these sections, and more importantly, getting them out of the way before tackling the actual comprehension questions. It's "money in the bank" 
The Bond English and Comprehension books are very helpful - watch out for the last Bond Comprehension book as it's very challenging and pitched more at the 13+ market. Athey and Alpha also do some nice comprehension practice, although the format of these and the Bond isn't the same as the CSSE and there's some stuff he won't need to cover. No practice is wasted, if you start fairly early.
Read to him regularly and get him to read to you - talk about the characters, settings and the action. What you're attempting to do, is encourage him to see beyond the words - to think about why the characters have acted in the way they did, "What would happen if "Mr X" hadn't said that?" all those kinds of questions. It doesn't have to be heavy and a chore, otherwise it might spoil the love of reading. You can start with some of his favourite books and then perhaps think about moving on to something a little "meatier" - more in keeping with past papers. Try to vary the type of reading and genres - fiction, non-fiction, poetry, newspaper reports from the broadsheets, historical and modern classics.
I could bang-on about this all day, with numerous tips etc but I don't want to bore anyone.