like stucknoue says, dd also found it helps if you are not intimidated by the fact that a lot of the other applicants are expensively educated (assuming your ds is not) and/or have previous experience you haven't
it's good practice: you will be working with people from all different backgrounds and it helps if you are the kind of person who can feel comfortable with that
you won't get the entourage thing or the child prodigies at somewhere like RADA and Central; as straight drama schools are very much about emotional maturity it is almost certainly better to turn up alone and try to present yourself as a functioning adult
what she also says (and this is from several years of doing the audition circuit round all the big drama schools) is that for straight drama there doesn't appear to be any particular advantage in having been at a theatrical secondary, so not to worry if some of the other applicants seem to think they are bound to get in because of that
further tips from dd:
choose audition monologues that suit your age and personality- at this stage they want to see you act, of course, but they also want to get a bit of an idea of who you are; don't worry too much about originality- but make sure you know the entire play!
be prepared to be asked to change your monologue and do it in a different way: they want to see if you can take direction
be friendly and polite from the moment you walk into the school- the receptionist and the students who take you round will report back to the admissions panel
if there is a workshop, throw yourself into it! have ideas- listen to the ideas of others! all theatrical training is about being able to be part of an ensemble
if there is an interview, be prepared to talk about why you want to train as an actor, why you chose that particular school (do your research on each one), and where you see yourself in 10 years time (be realistic but also make it clear you are very driven- if you aren't there's little point in applying)
also be prepared for the fact that many drama schools do not have halls, so you may be expected to cope with a certain amount of adult independence (teaming up with course mates to rent a house), and the facilities for student support may be much less than you'd expect from a university
Guildhall have a maximum no of times you can apply; the other schools will let you try again and again
if you take a gap year (with or without previous rejection), there is almost certainly no advantage in not working unless you are actually doing a theatre-related course. Anything you do can feed into your acting; dd worked in cafes and felt that was a brilliant opportunity to study how people move and talk; also to get her own confidence up around presenting herself