Once you've got a web-to-print application up and running, you will need to make sure that the end users keep using it. This may not be your responsibility as a printing company. But even then, you can guide the central marketing organization and provide them with tools.
Web-to-print home page
One of the primary tools to promote certain templates in your web-to-print application, is the application itself. And one part of it that all users will visit, is the home page. Consider adding some simple news items to this home page, each linking to an article about some new material. "The new posters have arrived. 10 tips to bring life to your store!".
Also, you might want to highlight certain templates in special ways on this home page. New items, promotions, the most popular templates in the web-to-print solution, and so on.
Wizard vs. Tree
Also think about the way you are going to visualize the templates for the user. Typically, two ways are mostly used. Either a wizard (with a number of questions following each other, finally resulting in a manageable list of templates), or a tree. The tree will contain the same structure for the templates, but might be more visible. A wizard is easier to navigate for inexperienced users, but it does not show the options in the other paths. And a glance at them might tempt the user to go in deeper.
Template Search
While it is not often used, you might also provide a search on top of the templates. In the cases where this is implemented into a web-to-print application, the users typically know very well what they are looking for. But of course you might incorporate some criteria into the search which are more accessible even for a wider audience (pulldowns with pre-filled types of templates, for example).
Email marketing
You shouldn't count on your users visiting the web-to-print tools on their own. So why not create a little newsletter about the application. Inform them about new promotions, new possibilities, or just about the fact that they have access to web-to-print solutions. Obviously this is a tool that should be used with moderation, but it can be very effective in getting the attention of your users.
Combined campaigns
And while we're at it, you might combine this email campaign with a physical mailing. You've got the tools, and you've got the addresses. So go and get the attention you deserve!