There were no pamphlets warning about US nuclear bombs dropped prior to the attacks on Hiroshima and Nagasaki. The notion that they were is a myth/misunderstanding that has unfortunately been repeated many times on the Internet (which, alas, does not make it true). Lots of details/backstory here, with somewhat authoritative conclusions on the pamphlet campaign's timing derived from this report produced shortly after WWII by the people who actually ran said pamphlet campaign. (And, if you still need convincing, see Richard Rhodes, The Making of the Atomic Bomb, page 737.) There were atomic-bomb-warning pamphlets, but they were dropped after Hiroshima and Nagasaki, not before. There were firebomb-warning pamphlets, which is a separate issue, and allegedly there were ones that mentioned Hiroshima and Nagasaki (I have never seen actual evidence of this, but it's not impossible), but that's a separate issue.

As for general precautions at Hiroshima and Nagasaki: there isn't much that I have seen that is authoritative on this subject. The Manhattan District scientists who went to both cities later said (and I haven't been able to confirm or refute this) that at Hiroshima they were actually planning to evacuate many civilians to the countryside prior to the bombing, but that said civilians were actually massed in the center of town on the morning of the bombing, in a grim irony. (If true, that's pretty grim. There is an element of it being a little too clever of a story for me to believe it without some better verification, though.) Nagasaki had some light conventional bombing during the war prior to the atomic bomb (contrary to yet another popular misunderstanding, it was not put on the "reserve" list that Hiroshima and Kokura were on that kept it from being bombed), as late as August 1, 1945, and apparently (according to another contemporary US government report) that led to some significant number of civilians being evacuated to the countryside prior to the bombing. But that too would need confirmation before really believing it, because it has its own potential motivations (e.g., underselling the civilian deaths, when in fact the Nagasaki bomb went off over one of the most civilian areas of the city, with military facilities only on the fringes of its blast damage).