This is a very personal type of situation to prepare for. Regularly what I do, say if I am having a nightmare where I am supposed to die, is I immediately do some Compassion/Lovingkindness meditation. Rosemary, when she's having a nightmare where she may die, starts doing reflection on the Buddha, Dhamma, and Sangha. These reflections come up automatically for us in our dreams, because we've practiced them so much. For me, it always helps take away the fear. For quite a few years now I actually haven't had what people call nightmares. I certainly had plenty of them as kid, every week the Indians would be around our house trying to kill us, so I know what nightmares are like. As I developed in the Dhamma and became more loving and kind in the daytime, like I said in the regular retreat, it carries over into my sleep and into my dreams.
So personally, you may want to ask yourself what is your favorite meditation? If it's centered around the Dhamma, basically in any way, it will be okay to use it around the moment of death. However it's going to come from your personality, as to what technique you favor doing at that moment.
Now, for Rosemary and myself, we prepare for this situation regularly, we train ourselves for it. Every time we take off in an airplane, every time we land, we meditate. Generally I do the Compassion/Lovingkindness for the world, a bit of how fortunate I am, and other reflections in that area. Rosemary often does the Buddha, Dhamma, Sangha reflection. We know that, by the law of averages, airplanes are most likely to crash during take off and landing. So we consider that we might die at that moment and we train the mind to think good thoughts at that time. That could be something you would like to start practicing.