For the 15min pose, its really important to get the full figure drawn, and to have good proportions, so my advice for that is to find out what the proportions are, and just practise them. It doesn't have to be very detailed.
For the 30min pose, detail is the key. Practise drawing eyes, mouths, noses, hair etc. Even practise drawing fabrics, clothes for the creases and stuff.
I've bought a few books about drawing people, to get the correct measurements, and to practise various things.
Hope i helped?! I'm spending my mid-term practising!!!!!