Well here is the second thing people do to screw up there web marketing campaign, not 301 redirecting their old domain to their new one!
So it turns out that your painfully long old domain name (e.g. www. toptenthingtoscrewyourcampaignrightnow.com) wasn't the right domain for your old website. Very often businesses will change the names of their companies or find a domain that is shorter/sounds better. This is all fine and dandy, especially if helps with your word of mouth marketing.
However more often than not some webmasters abandon their existing domain and just upload their new website to a brand new domain. Who cares about that old domain, it was silly idea right? Wrong. News flash: When it comes to search engine positioning domains age like wine! The longer your domain is around the more likely it will help Google gain trust. So what happens if you are not going to use it anymore? You need to permanently 301 redirect it. This helps you out two folds:
1. The obvious: Your return visitors from the old domain will be easily be redirected to the new domain
2. The somewhat obvious: the search engine crawlers will be able to find your new domain easily when they crawl your old domain (that is already indexed)
2. Not so obvious: 301 redirecting will help pass off Google Page Rank and Trust Rank to your new domain
Be careful here, many hosting companies such as Godaddy will offer an automatic domain forwarding service. There is evidence that this is not a true 301 redirect. When it comes to SEO don't take the risk, make sure ot have your webmaster create a permanent 301 redirect to the new domain.
Remember when in doubt, 301......