Hey guys, here's my advice (me being an experienced Olimpiada-taker ;))
1. Just be yourselves, try coming across as charming and smart, not as swots!!! Which means that if you do have a lot of knowledge on a subject you should be enthusiastic about it - you're not scholars yet, no need to keep a straight face :) Don't worry if they will seem to want to kill you; just be nice and charming and don't let them enjoy their (temporarily) superior position. And don't worry if you know nothing about a subject: they are not expecting you to know everything. You need to know the basics and they love it when students have fun facts prepared.
2. Tricky questions happen very often (they love them, really). Even the kind of: did you enjoy reading [a book]? -- when the book was a horrible postmodern text you coulnd't get through - then the answer is NO ;)
3. Do research on the books you chose (eg. read some critical responses, if the book is new - check out comments on Guardian if the book is discussed there http://www.guardian.co.uk/books (saved my ass)) BUT HAVE YOUR OWN OPINIONS AND DEFEND THEM
4. Don't panic, but kinda hurry in the written part - little time :( (last year it was 2h I think)
5. And as for other countries: Ireland is pretty important. Just know sth about potato blights :D Canada, Australia, New Z - not so much. Reportedly, a few years back one commitee massacred most of their students by asking questions about C, A, NZ almost solely - so it shouldn't happen again (and hasn't, since).
6. Do some sightseeing in Poznan, enjoy yourselves & good luck!!!