HTML

<div class="inner-addon left-addon">
    <i class="glyphicon glyphicon-user"></i>
    <input type="text" class="form-control" />
</div>

CSS

/* enable absolute positioning */
.inner-addon { 
    position: relative; 
}

/* style icon */
.inner-addon .glyphicon {
  position: absolute;
  padding: 10px;
  pointer-events: none;
}

/* align icon */
.left-addon .glyphicon  { left:  0px;}
.right-addon .glyphicon { right: 0px;}

/* add padding  */
.left-addon input  { padding-left:  30px; }
.right-addon input { padding-right: 30px; }