Say you want to vertically align a div box inside a div container. This is how you do it:

HTML

<div id="container">
  <div class="box">
    <span> Some text...<br />in two lines. </span>
  </div>
</div>

CSS

Set the line-height to the container's (implicit) height. The container MUST have a height >= its line-height, because the line-height actually spans the area inside which .box will align vertically.

#container {
  line-height: 50px;
}

Because the container's line-height is inherited by .box, reset it to a normal value now. You may use the (browser dependent) default value 'normal'. Omit display: inline-block; for elements that already are inline elements (e.g. a span).

.box {
  vertical-align: middle;
  line-height: 1.1em;
  display: inline-block;
}

Also see How to make a single check box (or image, etc) align vertically.