We are already hearing about the alleged "big bounce" that McCain got from his surprise Palin pick in 2008 but this is largely a myth. If anything, it set a record for brevity for such picks, most polls showed. And the even bigger myth was that it helped him with women. Many in media even claimed that it would help McCain get the votes of a lot of "Hillary Democrats."
In fact, most polls (as I covered at the time) found that McCain gain male voters--and lost women (and indies). For example, a CBS poll just days after the Palin pick found Obama with a widening 48% to 40% bulge--with women backing Obama 50% to 36%. The Palin picked actually doomed McCain, despite the big crowds she attracted (and remember some in crowds urging "Kill him!" re: Obama?).