Website migration is an exclusively technical process handled by the website developers or hosts. However migration can cause negative impact on search results, it can reduce traffic and therefore conversions for the website.

In order to manage a successful migration the following best-practice points should be considered:

            1. 301 URL migrations.
It is essential from an SEO point of view to ensure that content and URLs are mapped from the old website to the new website using a 301 permanent redirect.
Redirections should be carried out on a page-level basis, rather than a site-wide redirection to maintain internal link equity, existing traffic and a good user experience.  Extra care should be taken on well-linked and traffic heavy content pages. It is recommended to carry out a content audit to ensure all website assets have been properly migrated and positioned in the new website structure.

2.    Internal Links
All internal links on the new website should be updated to reflect the new structure.

3.    Sitemaps
Submitting an XML sitemap will help with speeding- up indexation.

4.  Site Crawl Speed
Once GWTs has been validated then it is good practice to slow down the indexing of the old website pre-migration and then speed this process back up post-migration (actually speeding up is less important than slowing it down - at least according to what Google says...). The same procedure can be carried out in Bing Webmaster Tools.

5.   XML sitemap
Post-migration is important to create and submit a new XML sitemap on your server and to Google and Bing as I already have mentioned in point 3. This will speed up the indexing of the new website structure.

6.  Updating External Links
As mentioned above, external links to the website should be redirected with a 301 header to ensure link equity continues to pass through to the new website. However, passing link equity through a 301 redirection loses ~10% value. Therefore it is best practice to audit the backlink profile of the website to determine the most powerful links in order to have these updated to point to the new domain URL.

It is also important to note that link equity can only pass through two sequential 301 redirects. Ensure that ‘chained’ 301 redirects are minimised to single 301 redirects where possible or updated fully to the new URL. All external links should be audited to identify chained 301 redirects.

What not to do when migrating domains..

1. Do not feel tempted to place 'Disallow:/' on the new domain before you place or redirection rules on your server. This will result in a significant visibility loss and will take you some good few weeks to build it up again.

2. Do not use 'splash' pages on the home page of the old domain. Even though it is nice to let the users know that you have moved or changed the name completely if that's the case its a bad idea from SEO point of view. Why? Because home page is usually the most linked page on your entire domain, hence placing any other redirect than 301 on it will result in all the link equity being not passed over to the new domain...You would be better off sending out a PR informing your audience of re-branding and website redirection rather than using splash page instead.

Here is the proof of why these two are a bad idea...

Vicibilty post migration

P.S SEO-girl can help you with sites redirections :-)