ALERT: sorry for very long post.
My advice is somewhat different from the others, but I expect there are many approaches and I am sure that all have their pros and also some cons, the point is we all care enough about our children's education to put some effort in.
I was lucky enough that my DD was always in the top set for english and maths at school, she was reading before reception and I have always tried to challenge her, mostly with reading and educational games. With conversation in the car about topics far and wide and mental maths on the way to school, etc. I believe she learnt a lot on our twenty minute walks to school. My DH is much better at times tables than I and he used to run through these all the time. Basically I just did this all naturally and always made sure she put in her full effort to any homework she was given. We were not thinking about the 11+ but just about her education.
Then at beginning of year five I began to think about some preparation to make sure my DD would know what to expect in the 11+ and to check if she had covered the types of mathematical questions that I had seen in the GL NFER practice papers, which I was amazed to find out, were exactly the same as the old set of NFER practice papers I had from 14 years ago!
My daughter began asking me if she could have a tutor, I thought she was doing well and told her that she was lucky to be very clever and really didn't need one and that they were very expensive. She continued with the parent nagging though telling me that all her friends had tutors. On checking with other parents I realised that she was right and did some checking on line.
I found a company that did 11+ lessons, these were not one to one but mostly one teacher to 10 or 12 children with one or two sixth form assistants. These only worked out at about £15 per lesson. I was happy that we could afford this as it was no more than some of her after school activities that she attended. She did one hour per week from about February but looking back I probably should have started earlier.
We worked on some Bond revision at home during the summer holiday as I was mindful that a long break might mean some regression so she had only a two week break when we went away on our holidays. On our return we had the missed lessons that we could do a free catch up on and with the same company my DD did a summer course and an 11+ mock test. If there were any sticking points I could message the teacher and he/she would go over it in her lesson. We then started doing some of the NFER practice papers just one or two a week and I began to reward her with 50p per effort put in at home and we put this in her special purse. We continued the lessons which she was committed to, perhaps because she had asked for them and we practiced right up to the 11+. In the last few weeks we covered test taking tips, such as working through quickly all questions then going back to ones that needed more time and educated guesses for all the rest.
I rewarded my daughter with the pair of kickers she wanted and some one direction paraphernalia straight after the test and not after we found out she had passed, at this time we just just gave verbal praise, lots of hugs and kisses and a cheap meal out. I believe in praising and rewarding effort whether she had passed or not I was very proud of her and the effort she had put in.
She passed both Bexley and Kent and missed the Newstead historical cut off by just two points. We could have still applied for Newstead and I will be interested to find out what the 2012 cut off actually goes down to by August. But we decided mainly due to ease of journey to apply just to all three Bexley Grammar schools and are very very happy with our choices. Finger crossed for March.
I hope this is of some help, feel free to PM me if your have any questions, all the best and very good luck to all your DDs and DSs.