There are a number of rewards and discounts that you could offer to your customers. Maybe you unlock free shipping at $100 or you give the customer a 5% discount at $200. Unless you use a lot of screen real estate to promote these rewards, most customers won't know about them or act on them. One way around this would be to list the different rewards and how close the customer is to reaching them. If you have good rewards, people will buy more products to reach the reward. This can drastically increase your average order value.
Set up free shipping so that it's unlockable at $100. This is covered in Chapter 5, Setting Up Shipping Methods. You will also need to install and activate the Cart Notices plugin, available on WooThemes.com.