The school should provide you with an outline of the expected standards for 7+ entry. For the north london consortium schools, they're all pretty similar, though obviously different standards will be expected.
Check she knows the tables she needs to know (2s, 3s, 4s, 5, 10s, most probably, but check the requirements). Practise a bit of maths with her (the Bond maths papers are good) so she gets used to facing different types of questions. Make sure she knows how to react under time pressure, ie. move on from a question if she gets stuck and then come back to it if time at the end.
For English, make sure she can write reasonably accurate and interesting extended prose. And maybe run a reading comprehension or two past her.
Though actually I did none of this with my dd2 -- when faced with the option of teaching her the 3 and 4 times tables during the christmas holidays, I'm afraid I thought, 'stuff it'. She got offers anyway (city of london girls and SHHS) but decided against it as I don't really go a bundle on the whole pushy vibe. She still got places at all the selective schools she applied for at 11+.
Is your dd happy at the state school? If so it's relatively easy to make up for the academic deficiencies at home.