Center Twitter Bootstrap 3 Glyphicons in buttons

Move the text out of the <span> with the glyphicon and apply vertical-align: middle; to the <span>

  <button class="btn btn-default">
    <span class="glyphicon glyphicon-th" style="vertical-align: middle;"></span> Centered
  </button>

Demo

This works in Bootstrap 4 with Font Awesome icons like so

<button class="btn btn-default" style="vertical-align: middle;">
    <i class="fa fa-times"></i> Centered
</button>