Do ask schools you intend to apply to about how best you can prepare your child – you are likely to be amazed by the disparity of advice you receive. Some schools will tell you, “Nothing at all.” Many will say, “Your child needs to read voraciously, especially literature.”
The most practical advice is: practice, practice, practice, especially at completing answers under timed conditions. Do ask prospective schools for sample 11-plus examination papers; these are sometimes available on school websites. A tutoring company will often have excellent resources to stimulate intellectual enquiry and extend a child’s enjoyment of learning.