This is a personality type question. Rosemary and I, each, have a different type of meditation that we favor if death is close. It's also going to be something that has to be cultivated regularly. You can't just think, "Oh, well, when I am close to dying I will do this one." Because when you are close to dying you may not have time to think. "Oh, I will do this one," death may be that close. So, you have to be ready, and to be ready very spontaneously means it has to be something you have practiced a lot.
For example, when Rosemary and I are on an airplane and we take off or land; since we fly so much, that is a time when we always get ready to die. We know that with airplane travel, although it is extremely safe, the majority of crashes come when it is taking off or going down. We get ready. For me personally, I'll do quite a bit of Compassion/Lovingkindness Meditation or reflect on how fortunate I am. Somebody else may reflect on the Buddha, the Dhamma and the Sangha to have unity with the teachings and the teachers when they die. So, it's a personality thing, but you want it to be something that is confidence-inspiring, something that gives you strength. This is because if you start thinking about negative thoughts when you are dying, well, it's just not going to help you. You want something that is very positive, and make sure it is something that you have practiced a lot.
One question very similar to this we had a year or two ago. It asked, "At the moment of death what should you think about?" Ok, this is it, here comes death, what should you start thinking about? I said that at the moment of death, it is too late. At that one last moment, it is too late to make a decision. So it has to be something that will come spontaneously very quickly.
Some people are able to judge whether they are getting prepared for death. It can often be in your dreams. For me personally, I've had dreams where I was going to die, I even had a double one once in the same dream. I was in a truck going along the mountainside, then the truck went off the cliff. This was it. Immediately I began Compassion/Lovingkindness meditation for the world in general. The truck hit the water and I wasn't dead yet. It went under the water. Now I am going to drown. My mind held to the thoughts of Compassion/ Lovingkindness, although I was supposed to be drowning. I woke up. It wasn't a nightmare. For somebody else that would have been a huge nightmare. They may have woken up screaming. But I woke up very happy because my mind automatically went into being prepared for dying due to my conditioning of so many years of meditation.
That can be one way people can see whether they are getting ready for dying. Another way to see this for some people is if they ever have an experience of becoming super sick and actually fainting. This happened to Rosemary once some years ago in a bathroom. If you can appreciate fainting in a bathroom, it's more dangerous than if you fainted right here sitting. You often have tiles and concrete everywhere. As she started to faint, her mind immediately focused on the Buddha, Dhamma, and Sangha, realizing that she might be dying. It was automatic because that's one of our favorite Meditation subjects. So train, train the mind in a confidence-inspiring meditation subject that can help you get ready for dying.