Just did this today! I used Click & Ship and even scheduled a pick-up for Monday. Plus, I had ordered my boxes and labels online (they got delivered last week), so I won't have to venture to the you-know-where anymore. Can I get an Amen? 
Per the USPS Web site:
Now use Click-N-Ship® to ship to U.S. military addresses around the world. Just choose "United States" from the country menu and pick the appropriate military designation from the state menu. You'll see the mailing restrictions and customs forms requirements for your package's destination. Postage is at the domestic rate and Delivery Confirmation™ is free with Priority Mail®.
After printing the label, you can complete the required customs forms online, too. (Plastic envelopes are required for the customs forms and can be ordered online through The Postal Store®.)
Use our Military Addressing Tips and FAQs to make sure your packages reach our troops