Be good at maths. Work experience and voluntary work are irrelevant unless they're directly related to maths (obviously they're good and you should do them, but they won't help your application). Further knowledge is good, but you can't really replace just 'being good at maths' -- Maths is one of those subjects where you can know lots of 'stuff' without knowing how to use it, but knowing how to use it is just as important. Know your Maths A-levels inside-out and more, try AEA, BMO and STEP questions (but only when you're ready of course), etc. Cambridge gives out lots of offers for Maths compared to other subjects because they know STEP will wipe out 40-45% of offer-holders, so in some ways getting an offer is the easy part!