What positioning and immobilization technqiues do you use for anal canal carcinoma radiotherapy in the era of IMRT?

On the heels of the discussions regarding skin toxicity prophylaxis and treatment, I am interested in your thoughts and current practice regarding patient positioning, immobilization, and reproducibility as it relates to toxicity. Do you currently prefer or use supine, versus prone, belly board vs nothing, vac bag vs nothing if supine, frog leg versus straight leg, vaginal dilator in females versus none, daily CBCT vs OBI vs less frequent imaging?