The prisoners run about before the selection process, trying to get some color in their flesh. The more color they have, the more healthy they will appear. This will ensure that they are chosen for work and not the crematory.
Also, when the selection process begins, the prisoners run by Mengele as quickly as possible. They want to appear healthy and they also are running quickly so Mengele cannot notice their numbers. Remember, the prisoners are known only by their number. If Mengele does not see their number, he will not be able to write down their numbers. In this way, the prisoners stand a better chance at not being selected for the crematory.
In one selection process, Elie Wiesel lies about his age and he tells the selection committee that he is a farmer. A farmer has a better chance at being chosen to work. In this way, Elie will escape the crematory for the time being.