This isn't as easy to do as one might expect -- you can really only do vertical alignment if you know the height of your container. IF this is the case, you can do it with absolute positioning.
The concept is to set the top / left positions at 50%, and then use negative margins (set to half the height / width) to pull the container back to being centered.
Example: http://jsbin.com/ipawe/edit
Basic CSS:
#mydiv {
position: absolute;
top: 50%;
left: 50%;
height: 400px;
width: 700px;
margin-top: -200px; /* -(1/2 height) */
margin-left: -350px; /* -(1/2 width) */
}