The most important thing when deciding whether to do the 11+ is does your CHILD want it? If they do, half the battle is won.
Visit the schools when your child is in year 4 or 5. This will help motivate them. How can they decide it's what they want if they haven't seen them?
Many many children are tutored for many many hours. I do understand parents desperation to get their child into one of these excellent schools at all costs, but for me, I wasn't prepared for my child to pay that price.
Having said that I believe with the system as it is, a child stands no chance of entry (no matter how bright) with no preparation.
Ideally start a child in year 4 on the Bonds books (available on Amazon) for an hour or so a week at home. Start with the book below their ability to give them confidence - they will be encouraged and enjoy it far more.
Talk, read, watch informative programmes together. It's not just WORK, WORK, WORK!
There are many free resources available on the internet and past papers are available from the CSSE. In total you may need to spend Â£100 which is less than many spend on 1 weeks tuition!
Do mock days for at least two months in the run up to the actual test. Many children are completely thrown on the day by the change of environment. Prepare them for it, even if you can't afford to do the organised mock days send them to a neighbours house or Granny's to sit and do the tests consecutively in silence.
Inform yourself before you start and make a plan of how you can support your child to achieve their goal. Make decisions for yourself not based on rumour and speculation.
Finally, this forum is fantastic and has been a constant reference point for me throughout our 11+ experience.