Gyaku-hanmi sokumen is our most common warmup technique. Or suwari-waza ai-hanmi ikkyo-ura. I have felt that these two, when done correctly both stretch and warm-up. So in the advanced class we sometimes do not do any stretching nor warming besides those two techniques. And if you're warmed up then you get started (and hot not warm:P) with hard kokyo-nage's.
Just my 2 eurocents...