You've just made a major redesign of your website, maybe even switching CMS platforms, and your URLs are going to be entirely different from their past iteration. Or, perhaps you have an ecommerce site, and you are making changes to your product line. This means that you are going to need to redirect any old URLs in order to preserve the content for which they were intended, or to keep visitors on your page. Do this correctly and your visitor won't notice any change, and you will have minimized any loss of browser organic traffic. Maybe even more importantly, your SEO ranking won't take a hit.
Common URL Redirect Mistakes
You might not be aware of it, but there may be multiple URLs for your homepage, especially if your new site was under construction for a while. They might look like http://myhomepage.com, www.myhomepage.com, both legitimate variations on a theme. Unless you tell browsers which variation is the preferred one, they will accept them all. This could wreak havoc with your SEO. Google has a set of guidelines for establishing canonical URLs.
File naming structure
Avoid, if at all possible, altering your file naming system. Just because you've got a new CMS is no reason to ditch file names which have already been indexed by search engines. That would be counterproductive. If the URL was http://www.olddomain.com/FAQ, then your new page should be found at http://www.newdomain.com/FAQ. If the spiders could talk, they'd thank you.
The META redirect
Redirects should be done in the server, but situations may arise when that isn't possible. Using HTML you could insert a redirect using the meta element with the value of the http-equiv attribute set to Refresh, and the value of the content attribute set to zero seconds. Theoretically, this instantly redirects visitors from an old page to a new one. Unfortunately, not all browsers will recognize META Redirect tags, so they should only be used as a last resort.
The 302 redirect
A 302 redirect is often a default, simple type of redirect in some CMS platforms. It also offers zero SEO benefit and can cause confusion for search engines, since old pages are retained and your content appears to be duplicated. Choose a 301 redirect, instead.
The 410 page
Technically, this is not a redirect. This page tells search engines that the page is gone, there is no substitute for it, and it will not ever return. Imagine if you are an online retailer who once offered a few hats because your brother-in-law had a hat company. Once your sister divorced him, there was no reason to carry his lousy hats. Best practice with this page is keeping a few 410 pages, if necessary, for a few months as a courtesy, then dropping them. Don't let them pile up, however. If you find that you need a lot of them, convert them to a 301 redirect.
The best redirects are performed instantly by your server in the backend, when no one is even remotely aware of any redirect taking place.
This directive advises the page has moved permanently, but the directive can be changed at any point if you need to update the redirection. This redirect is the best practice to retain any SEO authority from an old page to a new page.
The 404 redirect
This is a perfectly normal page on a busy website. Think of the 404 redirect as a way of saying that the information has moved with no forwarding address. Provide a custom 404 page which allows the visitor to either return to the home page, or offer a search box so that the visitor can enter a few keywords that will hopefully get them to a desired destination.
Any time you shift a page from the old URL to the new URL, even using a 301 redirect, it takes the search engine spiders time to figure this out. If the old website or CMS had poor traffic, this process can take even longer. SEO ranking will likely be affected for (hopefully) only a brief time. Be patient.
Also, if you have more files to redirect than Beyer has aspirin, and you don't have the time to redirect everything from the old domain name to the new one, you should at least redirect all the files that have been indexed by the major search engines. If you must be selective, do it carefully.
Photo credit: Shannon K / Foter / Creative Commons Attribution-NonCommercial-NoDerivs 2.0 Generic (CC BY-NC-ND 2.0)