One of the things I noticed during the recent VK TT course was how useful certain aspects of my ashtanga background have been, in particular the way we tend to work on the same poses day after day. In many yoga classes it seems, the poses and sequences can change from one class to the next, thus a posture you find difficult might not come around again for a couple of weeks. In Ashtanga, if your last posture is laghu vajrasana, you'll work on on it five days a week until you nail it and once you do you'll continue to polish it day after day. It's a possible drawback to Vinyasa Krama in that your encouraged to try to cover as many postures as possible over a week or a fortnight, although you do have a couple of key asanas that your advised to practice everyday.
My way around this is to practice the key asana as advised and then choose a couple of challenging asana to work on, if not daily then every other day, the rest of the practice will then rotate through the different sub routines and sequences over the week. The 'challenging' postures (to me anyway) that I try to work on everyday are Natajarasana, Hanumanasana and Eka pada Raja kapotasana.
One of the difficulties with Natjarasana and Eka pada raja kapotasana is the grip on the foot. here they are at full speed and then in slow motion.