T.S.
My husband served in the navy for five years. He was a CT so we did not have as many separtions as most Navy couples, but we did have a few. One silly thing I did once was spray my perfume on a letter I sent to him, I thought it would be funny, but he loved it.
I think the biggest thing is allowing them to be involved in your everyday life so don't feel left out and they can imagine themselves home. How about taking a friend to your next OB appointment and having them record the heartbeat or take pictures of the doctor measureing your belly. You could post a video on u-tube for him to watch and email him the pictures so he doesn't miss out on all the neat new things that come along with your first baby. Send him pictures of any neat new things you buy, like the crib, etc. Or better yet, email him photos of choices so he can be involved in the selection.
When the baby is born send him another video once per week and lots of photos. It will be something for him to expect and look forward to during the week.
My brother is also in the navy and works on subs. They surface once per week to get emails, and photos take up a lot of "bandwidth" so posting on a site like myspace or photobucket may make it easier for him to view than if they were attached to an email. Also, Walmart and Kodak both offer free online photo albums that are completely private.