Hi PeppermintScreams:
Wow - not sure what part of the world you're in and I can't claim to be a great success (DD1 was close, but didn't quite make last year's cut-off so it's not looking likely) but this is my advice:
Y1/ Y2 (KS1) - Make sure that they leave KS1 with sound reading skills (aim for free reading end Y2).
Make sure that they leave KS1 with sound skills in numeracy: so able to add and subtract at least up to 30, but preferable up to 100. Able to count by 2, 5 and 10 (effectively know those times tables - but not necessarily realise it).
Y3/ Y4 - READ! Really have them read proper fiction (DD1 really didn't get there until Y5 - and this is what I regret. Y3 may be a bit early, but by Y4 they should be reading things like Roald Dahl (Charlie and the Chocolate Factory, Fantstic Mr. Fox, etc...), Michael Morpurgo (9 lives of Montezuma, Animal Tales) or Classics like Charlotte's Web and Babe (The Sheep Pig is the original title).
Around here (midlands) many start tutoring in Y4 (especially if they have doubts about core skills - numeracy or literacy) but most start seriously with Bond Books/ Practice Exams/ Mocks in Y5.
If you haven't discovered it the eleven plus forum has information by your area of the country and that's a good place to start. At the top of each regional forum there is usually advice on how to prepare.
Finally, as a Mum who's DD1 didn't quite succeed - especially if this is an area where competition is fierce - please please consider making this about gaining sound skills for the move up to senior school (so they'll do well wherever they go) rather than about pass or nothing.
HTH