The customs form requirements are discussed in these forums ... somewhere (too early for me to go looking) but the most important thing to remember is that there are two different forms - the small green one and the larger white one. You want to use the larger white one. You can pick some forms up from the post office to fill out ahead of time, which is a good idea because it gives the clerks the impression that you know what you're doing and keeps their questions about the contents to a minimum.
Be sure to mark the package as "gift," and when it asks for contents, experience has shown it's better to describe it as "plastic model kits and hobby supplies" rather than getting very specific.
It's also a very good idea to write "Forward to unit chaplain" in the lower right corner where it asks what you want to do if it can't be delivered to the sender. That way your box won't end up either rotting in a mail room or, worse, getting sent back to you.
I always enclose a letter on top of the contents with the address plainly written in large type, just in case the address on the box somehow gets messed up.