As Mike points out, one way to get people to switch is to be obviously better. That’s what Google did to Altavista to steal the crown.
No, that’s not true. The real reason Google is popular is because Yahoo! switched from Inktomi(in a way, AV) to Google as their search engine, complete with “powered by google” signage. So you have the #1 website on the Interweb telling everyone that Search = Google. Then, when search became important (because the Internet exploded in terms of content, and hence usefulness of web search), people had no trouble to switching from a huge portal to a dedicated search engine they already used.