You can play width top/bottom properties having margin prop in "auto";
If you have a block like this:
<div class="centered"></div>
It may be centered verticaly like this:
.centered {
width: 100px; height: 100px; /* must be present. No matter the value */
position: absolute; /* to props top/bottom take effect */
margin: auto; /* here's the magic. */
bottom: 0px; top: 0px; /* This is how you center a block verticaly */
}
The same can be achieved for horizontal alignment width left/right properties. You can also have an offset in order to ubicate other point than the center of the block.
Here I leave you some examples of what and how can be done combining these properties.
http://jsfiddle.net/SQDJ6/
