Where is the proof for von Däniken's claims? Some of it was fraudulent. For example, he produced photographs of pottery that he claimed had been found in an archaeological dig. The pottery depicts flying saucers and was said to have been dated from Biblical times. However, investigators from Nova (the fine public-television science program) found the potter who had made the allegedly ancient pots. They confronted von Däniken with evidence of his fraud. His reply was that his deception was justified because some people would only believe if they saw proof ("The Case of the Ancient Astronauts," first aired 3/8/78, done in conjunction with BBC's Horizon and Peter Spry-Leverton)!
However, most of von Däniken's evidence is in the form of specious and fallacious arguments. His data consists mainly of archaeological sites and ancient myths.
He begins with the ancient astronaut assumption and then forces all data to fit the idea. For example, in Nazca, Peru, he explains giant animal drawings in the desert as an ancient alien airport. The likelihood that these drawings related to the natives' religion or science is not considered. He also frequently reverts to false dilemma reasoning of the following type: "Either this data is to be explained by assuming these primitive idiots did this themselves or we must accept the more plausible notion that they got help from extremely advanced peoples who must have come from other planets where such technologies as anti-gravity devices had been invented." His devotion to this theory has not dwindled, despite contrary evidence, as is evidenced by still another book on the subject, Arrival of the Gods : Revealing the Alien Landing Sites at Nazca (1998).
http://skepdic.com/vondanik.html