Of course, you don't want to overlook the most obvious, and most powerful sales technique in any industry: impulse, add on items.
For example: Your customer orders 72 shirts with their logo. When they receive the goods, they open the box to find not only their shirts, but also a hoodie with their image printed on it, as well as an embroidered or digitally printed cap with their logo on it. In the box, they also find a very simple price list, as well as a "quick order form" to make it as easy as possible to convert their sense of "Wow, this is neat!" into "I'll take twenty."
It might be a good idea to include a SPECIAL PRICE, found only on the price lists in orders you send out, which is good for 5 business days from the day they pick up their order. You could have a small field on the forms, where you stamp or write the date, so you know how long the special lasts for each customer. This would add a sense of urgency to their desire to buy a few hats, sweatshirts or alternative items.
I find that, most of the time, people don't order more variety of products, simple because they never take the time to conceptualize their image on anything other than a t-shirt. If you SHOW them their logo on a sweatshirt, they are more likely to buy them than if you simply offer them.