[jdiehl]

I would think that the easiest way is to implement a coupon. You can setup a coupon that gives free shipping based off the coupon code being used, and only valid if an order is over a certain amount.

Then you can advertise the coupon as something special with a deadline. This creates urgency from the customer to take advantage of the special before time runs out. Chances are every month you are going to create another new coupon code with free shipping again and update your advertising, but it gives that additional extra push for customers to buy before the end of the month. I've seen it work in a few stores, it depends on your store I guess.