What is the best way to merge websites into one without completely messing up any SEO?

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

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  Clayton Nichols: I would advise against just shutting down the blog cold turkey. If you wanted to. Create those articles on your new site and 301 redirect all of those old URLs to the new respective URLs of that content. ;
  
  Doing a 301 redirect to the new content still pushes through some of the link and authority of backlinks that were attached to those old domains/URLs.
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  Kevin Vitali: Thats what I was thinking. ; I was going to move the posts slowly with a 301 redirect and then leave in place for 6 months or so....
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  Clayton Nichols: Yea the pacing of how fast or slow you move doesn't necessarily matter. Just make sure that you're not leaving those pages down long or anything like that. ;
  
  I may not even hurt to place a canonical tag on those pages that are going down pointing to the new content prior to removal(that's ambitious though).
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  Kevin Vitali: Might be ambitous, but certainly over my head +Clayton Nichols LOL
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  Chase Billow: If it's a LOT of pages you want to move, don't move them all at once. If it's not a big deal just move them over and 301 everything and you're good to go (assuming you're keeping that old domain).
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  Tony McCreath: For as long as the old domain has backlinks you want to keep it running with the redirects. So once you've moved it and set up those redirects consider doing some outreach to get people to update their links. It's also an oportunity to connect with the people who link to you.
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  Clayton Nichols: Agree with +Tony McCreath. The cost of paying for a domain is cheap enough and better than losing quality backlinks(if you have them).
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  Kevin Vitali: Thanks all for your help 
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  Luke Ciciliano: Webmaster Tools has a new feature that allows you to move all of your content to another domain and, if you follow the process, Google will see your site as having moved to another as opposed to just seeing the second site as duplicate content. There's a procedure for this in Webmaster tools that you may want to check out ;+Kevin Vitali. It's actually Google's preferred method of moving content. ;
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  W.E. Jonk: From the expert panel in this weeks SEO Questions hangout on air on 01:27:06 into the YouTube video: https://dumbseoquestions.com/q/what_is_the_best_way_to_merge_websites_into_one_without_completely_messing_up_any_seo