Like with domain names, once a desired username is gone…it is gone*. Thus, you should seriously consider registering your personal (or preferred) name on the most popular social media sites (just in case you decide in a few months/years to start using the site).
You should also consider encouraging your clients to register the usernames consisting of their trademarks. You can even plug in their brand names into the handy search engine at UserNameCheck.com and see whether nor not third parties have already registered them.
As a big fan of Twitter, I’d suggest starting there…
*Note: most sites have provisions for trademark owners and celebrities to obtain their own names/marks back. For instance, Twitter’s terms of service states that “We reserve the right to reclaim usernames on behalf of businesses or individuals that hold legal claim or trademark on those usernames.”