The first thing to consider is 301 redirect. This is very important especially when there is a need to change a link from the new design to a new one. 301 redirects also ensure that search engines will know what the replacement of the old link would be and able to index them instead.

The next thing to do is to redirect all traffic from non-www to www or vice versa. This is because search engines find that www and non-www to be two different sites. If there aren’t any redirects, it may cause search engine to penalize the two sites due to duplicate content, etc.

Create unique meta tags for each pages so that it will avoid any duplicates. Think of the new pages as a new way of getting keywords opportunity.

If there isn’t a need of using Flash, don’t use it. Not only does the search engines unable to read the content in Flash but some users find Flash a nuisance especially legacy computer users. This is because Flash takes up a lot of computer resources that may cause hangs in their computer. The last thing a website needs is visitors holding grudge on them because of Flash contents.

Remember the 5 seconds rule: if they cannot find any useful information in 5 seconds, they will most probably bail out from the website. Keep this in mind while redesigning the website.