Short as possible, relevant as practicable and make sure that the client approves before you grab the registration.
If $company.com is taken, sometimes we do $company$city.com, or perhaps better, $company-$biztype.com.
Also, initials Big Fat Company, Inc, can be BFCI.com. [edit]be sure to just check for "bigfat.com" also....[/edit]
Of course, the namespace there is pretty well-traveled. I've got a .biz or two also.