In decades past, there was a tradition for parties to unite by selecting the runner-up as the VP candidate, however that has gone by the wayside in more recent years, particularly with candidates who genuinely appear to dislike each other (such as Reagan and Bush, the last example of choosing the 2nd place finisher) being rejected in favor of candidates who can appear better unified, particularly since the selection of Al Gore -- Al Gore seemed like an poor VP selection at the outset (since he overlapped with Clinton in so many ways, with both being southerners with similar political positions), however he and Bill Clinton campaigned well together and appeared to genuinely get along, and he has since been viewed as a savvy selection.

To some degree, Sanders's open dislike of Clinton has probably eliminated his chances of being selected -- it would widely be perceived as a purely calculated political move, and that's exactly the image Clinton needs to avoid.

Instead she'll likely follow the more common trends of recent decades. Today, VP candidates are usually selected to either (a) shore up a perceived weakness of the candidate (Biden being selected for things like his foreign policy experience) or (b) targeting a contested state or demographic (Palin being selected in the hopes of targeting women upset with Obama beating Clinton). For the former, she may choose a strong progressive to appeal to Sanders supporters, though Sanders himself won't be the choice; Sherrod Brown or Elizabeth Warren would be more likely. For the latter, a Latino candidate or a swing state candidate would be most likely (Sherrod Brown could also fit this, as he's from Ohio).