What the others said about 301 Redirects is correct, but you should make sure nothing is left out. I find it is better to plan ahead as much as possible to avoid errors which could affect your rank.

Using free tools like SEO Spider can help you figure out all the links on your site that point to images, css and js files, etc.

There is a nice checklist you can use at http://moz.com/blog/achieving-an-seo-friendly-domain-migration-the-infographic that covers it.

Best of luck!

set up a permanent 301-redirect at the server level from the old domain to the new. That is pretty much it.

In regards to the correct implementation of 301's - Always content match old to new urls as much as possible.

When making a switch to a different domain, what I personally like to do for SEO purposes is build the second site on a different domain. So if your primary domain is a .com, I would build the second site as a .me .net etc.

Instead of completely shutting down the original site, I usually do a redirect and keep the redirect until my second site gets visibility on search engines.

Also, most of my sites are built on Wordpress. I find Wordpress to have the best out of the box SEO and allows quicker visibility on Search Engines.

Make sure you 301 redirect your links if you change the link structure during the switch. Keep an eye on Google Webmasters for anything you might of missed.

301 redirect is the obvious answer and making sure everything matches is essential. There is a good chance you will take a hit on SEO for a short period of time. Dont panic. If you did the job correctly it should come back rather quickly and actually rank higher if you know what you are doing. You need to create a target rank and figure out how to beat out those ranking higher than your site.

I'll be honest, many of the answers to this question here are far too simplistic and could be dangerous (for your site) because of the lack of information.

I've done about a dozen of these and each time it's complicated and something is missing. Here are the questions I've learned to ask.

Yes, 301 redirects are important between the old pages and the new, but digging deeper in that question you have to ask:

-Is just the domain changing, or are all of the URLs changing as well? For example are you going from oldsite.com/product-1 to newsite.com/product-1 OR are you going from oldsite.com/product-1 to newsite.com/products/shoes/product-name-32454?
-Is your whole website URL architecture changing? With whole categories of pages being added or removed?
-Are you moving your blog from a subdomain to your root domain or vice versa?
-Do you have ten pages on the old and new site, or tens of thousands? That will impact your method of redirects and how it affects site performance

Beyond 301 redirects, other things to consider:
-Can you isolate changes on your site into different stages? For example, just change the domain name and nothing else in this stage. In other stages you can change the design and content. Often times people want to go whizbang all at once and do everything.
-Are you switching platforms, such as a new CMS or Ecommerce platform? That changes a lot
-Are you dealing with parameters and filtering? Magento and other platforms create thousands and thousands of these URLs that you have to make a decision on
-Do you have a competent developer that will lead you, or do you need to triple check everything when it comes to SEO?
-Are you making sure all tracking codes make the leap between sites?
-Have you checked if the new platform has the same ability to edit important elements such as the title tag, meta description, URL, meta robots, rel=canonical, etc?
-Are you making sure that each page has it's own URL and you're not moving to a tab-based page view that will affect your page count? (Still matters)

This is just a sampling of some big important questions, and it's different for every project! Make sure to plan it better than you thought you'd need to, at least 30 days in advance.

Happy to answer any followup questions!

My advice is to update all physical branding materials that mention your old name or domain for example business cards, PowerPoint templates etc.
Besides if you do have any questions give me a call: https://clarity.fm/joy-brotonath