Not sure if this has been said already but its always a good thing if you have 4 of you, 2 over 18.
Get the 2 18's with ID to go in and pay for 2 tickets each, as my birthday is in november (despite going to a couple of gigs and being accidently given the over 18 stamp cant do it at cinemas who ID you if you look under 25) *shakes fist* I've been the "girlfriend stuck in traffic" for a couple of 18 films, you then pass the tickets out outside, go in and since i dont look 12 you get away with it fine, just dont look like your about to cry when your tickets checked, start a conversation with the cinema staff, usually their pretty awesome as half of them are students anyway so wouldn't care even if theri noticed.
As for buying tickets for other films don't, cinema's catch on to this and end up posting individual people to check tickets on the doors, aswell as coming in from time to time with night viewing aids to count up seat and ticket numbers and so they can spot the 5 year olds sitting in the middle row :P