This is probably not the most usefull advice but start praying!!
More seriously though, the best way I find to do this (long term) is listen to lots of 'classical' music, read up on all the periods, forms etc, and find someone who knows what they're talking about and discuss it pieces with them.
On the short term is harder, the only thing I can suggest is listen really carefully to the piece of music ,(
don't do what I did and slip off into a day dream while the examiner is playing) say something even if it is a bit gibberish, if you don't know what it is, say what it isn't (This seems to work, cause I don't know how I passed grade 6 aurals if it doesn't) eg if you don't know exactly what period it is from, but know it isn't early, say so. Oh, and for saying what time signture it's in, remember to count while the examiner is playing the piece the first time, working it out from the extract is almost impossible.
Seeing as you are a singer, I don't suppose you can give me any advice on the other parts of the tests , my singing tends so have this unique style also known as strangled cat, etc...