How do you mitigate the risk of echo chambers?

Encourage skepticism, but avoid that echo chamber. Echo chambers in society, like their physical counterparts, require selective feedback to re-inforce whatever meme has a person/group gripped in their resonance. A healthy skeptical/analytical POV can reduce the 'feedback', in much the same way as foam coned walls attenuate the feedback in an echo chamber = an anechoic chamber. This means that search engines must have a source SEO that is counter to the particular meme being put forth as an echoed belief's web site(which has its own SEO to present it's filtered meme to it's adepts at all levels) We see this war every day, so encourage skepticism, learn how to detect the false skeptics who try to add volume to the 'echo'. Most, if not all, religions want closed echo chambers for their beliefs, sadly they are able to gather huge numbers of people - often by killing skeptics in some religions, so be wary, cover your tracks, guard your privacy.

An issue I've seen becoming more a reality lately is that echo chambers are becoming self-sustaining. Moderators on popular discussion sites are becoming more and more emotionally driven, and maintain the status quo to the point that you can predict what you'll read and see well beforehand.

The above being strictly an online issue, the answer I'd have: spend less time discussing things online, and more time doing it in real life.