Help using options_for_select in select_tag

<b>Client</b><br />
    <%= select_tag 'clients[]',
    options_for_select(
    [[' ','1'], ['aaaa','2'],
['daemon','3'],['nortel','4'],['telefonica','5'],['local','6'],['otro','7']]
),
    { :multiple => true, :size =>4 }
  %>
  </p>

Instead of using the ['...', '.'] code I would like to use a method of a
list of clients that I have.

options_for_select(@clients.map { |c| [c.name,c.id])