There are many ways to create a search box on your site. When you want to dedicate as much screen real estate to your site’s content, a search box can often distract by taking up too much space. Also, if you are running a few sites, re-styling elements takes time and having a search box means there is just another element to re-design.
What I really like at the moment are full screen search boxes. How they work is that you have an icon or a link and when you click on it, the entire screen is taken up with a simple search window.