The following are some of the stretches I do with my dogs. There are certainly a lot more that you can do, but this generally covers most of the body.
Active stretches can be done before running them in agility, but you should avoid passive stretching until after.
Stretches are so important to keep the dogs flexibility and range of movement. I often just do an easy conditioning session, finishing off with really good stretching the day after agility trials too.
The main thing with stretching your dogs, is that you don't push them past where they are comfortable. If they are tense while you are stretching them, then you won't achieve much anyway. That's why I explain in the videos, how you can help teach them to relax the muscles themselves and give in to the stretches. A few of my dogs were never good at relaxing so it's helped my guys heaps.