I suppose I also mean this more generally for other universities, but in particular, for Oxbridge.
My feeling is that because, quite important parts of the admissions process come down to how you perform on the day (which may be unfortunate or fortunate) - the admissions tests, the interviews - this means that it's very, very difficult to coach someone to get into Oxford or Cambridge?
I also think that getting a place is down to the applicant's ability to think critically, how prepared he or she is for defending why they enjoy their subject and why they want to study it at university and how good they are at their subject. But it's still up to chance IMO.
PS I went to Cambridge - didn't get in when I first applied (had an interview from hell with a philosophy professor - wasn't even applying for philosophy!!) and had much more positive interview experiences the second time around when I re-applied.