Several ways.

If the div is going to always be a fixed width and height the best way is to use a background image for it. It could be included in a CSS Sprite image and would be quicker and more semantic.

If the div is going to be dynamically sized based on content then you could use multiple images for each corner and position them using CSS inside the box.
Or you could use javascript to do it for you which would keep the code clean for non-javascript users but it isn't as quick as non-js methods.