We consolidated 6 sites into one just this year as part of a re-branding effort. We took several steps:
1. The message that the business was becoming a larger whole was shared with the current clients over the course of 6 months to a year before the migration via email messages, letters, and press releases.
2. The week we switched over an email about our 'new' name/brand was sent to everyone for whom we had an email address.
3. All of the old domains were redirected to our new site, but not just to the home page of the new site- we set redirects deeper into the site to the content that best emulated the content they would have found on the home page of the old site. (Done via http redirects on the home page using php on the referring url.)
4. The redirection of the old urls was set to indicate to search engines that this was a permanent move, not a temporary redirect.
5. We're working on SEO for the new site using all of the old keywords and improving on what wasn't a very good optimization strategy for the old sites.
Something we missed, and probably should have done, was post a message about the upcoming switchover on the old sites-- our focus was on current site users moreso than the casual visitor. They wouldn't have the rug pulled out from under them with a 404 error upon hitting our site, and should get to the same general info they'd seen before, but the look and feel of the site would be very different which could lead to confusion.