Up to the time of the Hiroshima attack, these leaflets were fairlygeneric: Expect massive bombing in these cities in the next day or so. Get out in time, save yourself -- that sort of thing.
Right after the Hiroshima attack, there was another leaflet campaign that informed people that a new type of extremely destructive bomb had been used, and urged them to ask their leaders about Hiroshima. I have read that there was a considerable effort made to confiscate these leaflets.
There were leaflets specially intended for the Kokura attack (which was aborted and shifted to Negasaki). Because of a scheduling mistake, they were dropped after the Negasaki attack.
During the later stages of World War II, Japanese civilians did not have a great deal of freedom to simply come and go as they pleased. I suspect these leaflet campaigns accomplished very little. Most of the citizens of targeted cities were probably unable to do much to save themselves.