I think you have to be very clear about who you are advertsing to, and what the particular information is that they might not already have.
So a general 'come to our open day' with Photoshopped pictures of smiling pupils on some 'untargeted' media - whether print, radio, or back of buses - would simply have me thinking 'that school is really desperate, I wonder why?', and so would be negative rather than positive.
A targeted method - e.g. leaflets in primary school bookbags (or, increasingly, e-mailed out to parents from an initial high-quality mail to school administrators) - that focused on Y7 in particular, publicising open days for Y5 and 6 parents, emphasising e.g. recent Ofsted, change of head, much better GCSE results, new sports hall, feedback from current parents on excellent pastoral care, 'now your child can stay on to 18 in our exciting new 6th form' would be much more likely to have me looking round.
If what you are really wanting to do is to attract current Y11s in other schools to apply to the new 6th form, then that needs a different strategy, and probably completely separate Open Days. You may well need to enlist your current sixth form pupils and their parents to discuss how their peers receive information - could well be a social media approach rather than anything traditional. Current 11-18 schools are very unlikely to be willing to circulate information that persuades their pupils to move elsewhere, so you will need to be cannier and more direct.
Through all of this, ensure that your website is impeccable. If you are making a 'thing' about the sixth form, make that either a very distinct section of the website or even a separate but linked microsite. Consider your presence on other social media - Facebook, Twitter, Youtube etc etc, and update all of VERY regularly (get current pupils to help, but retain overall editorial control as the worst thing would be to have poorly spelled Facebook entries on a school-branded page). Again, consider separate accounts for the sixth form if you are pushing that, and make that all about sixth form doings - prospective sixth forma applicants don't want to know about the U11 football team, but they might be interested in the sixth formers who won an engineering competition / raised money for a local charity / did a really exciting local internship / had responsibilities / experiences within a small sixth form that they wouldn't get at a larger one.
Be REALLY clear about what your selling point is - as a previous poster said, it may not be what you think it is!