URL parameters are kind of crazy due to their faceted search functionality

Selected answers from the Dumb SEO Questions Facebook & G+ community.

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  Alan Bleiweiss: I deal with this several times a year in my audit work. There are several factors and considerations, where it can get complicated. However there are certain standard rules in most situations. 1. Someone needs to decide WHICH facets deserve to be indexed, if any. This requires understanding organic search volume coupled with revenue data from ALL referrer sources, and finally cross-referenced to Profit Margin (some revenue is so low profit as to not be worth increasing visibility for). 2. With that data in mind, you still need to recognize that the scale of potential, direct or perceived duplicate content generally means it`s best to only allow a very narrow set of facet combinations to be indexed. 3. All other facet types need to have the end URL set up to ensure they have a Canonical tag within their headers that points to the non-faceted version of the URL for the primary category. 4. Those same URLs, when parameters are present, need to be checked to ensure they do NOT have a meta robots "index" status. 5. Google Search Console needs to be set up to ensure it`s NOT set up for "Let Googlebot Decide" with such URL parameters when those parameters do not deserve to be crawled OR indexed. 6. Sitemaps need to be set up to ensure those URLs you DON`T want indexed, are NOT in those sitemap files. 7. Even with all of that effort, some URLs may get indexed in spite of canonical tags pointing to root variations. If that happens, you may need to block parameter indexing through the robots.txt file. Yet you need to be VERY cautious so as to not end up blocking content that does deserve indexing. NOTE: There are ways to allow more facets to become indexed in a way that doesn`t harm SEO, however those ways require a big "it depends" answer that is way more complex than can be easily stated here without comprehensive knowledge of the site or other factors.
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  Michael Stricker: Then, one day, client moves to a new platform or cart, and life begins anew!
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  David Kutcher: Pre-fetch all query permutations, assign them each a unique ID to be aligned with a canonical url. If a permutation is not in the db, add it and a new canonical as part of that search and render.
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  David Ogletree: Sigh. URLs have nothing to do with SEO. Use canonical tag. Move on to getting links and writing better content.