1) I've always pronounced ου as [u], as Mastronarde says, but now I hear that in classical Attic, it was actually pronounced as [o:] or as 'A Companion to the Ancient Greek Language' puts it, [ọ̄]. I'm not sure what they mean by the dot underneath though.
So δηλοῦν would be [dɛ:lo:n], not [dɛ:lu:n].
This changes quite a lot for me. I've already stopped pronouncing ει as the diphthong [ei] and made the transition into [e:], but now I've got to revise my pronunciation for this as well!
2) Do you pronounce υ as [y] or [u]? I've always pronounced it as [y], but apparently the change from [u] into [y] was a relatively late phenomenon.
3) I'm fine with the aspirated stops because I purposely exaggerate them (and it's fun to do). But I have much more trouble stopping myself from aspirating the unaspirated stops. I suspect an Attic speaker would find that instead of τιμάω, I am saying θιμάω more often than I'm aware of. Damn my engrained English phonology!
Here's a tip though. We don't aspirate our stops in English if there's another consonant before it, like [s]. For example, strain vs train. I say στιμάω a couple of times, and then phase the sigma out. Voila, an unaspirated stop.
