Write letters, send clippings, flowers, comics, things that remind you of him, books.

You can even write a whole bunch at a sitting and either mail them over time or send them all at once, but numbered so he has to open them one a day or one a week, like an advent calendar.

There's nothing like physical letters.

You should also get him to send nude photos. He can send them to me, and I will forward them straight to you after vetting them so you only get the best.

IMing can be fun, and can be done while doing computer work, browsing humor sites, or catching up on news.

Phone calls are awesome when they're special - don't call multiple times every day, but talk on the phone a couple times a week, and set aside actual time for it when you won't be distracted by other things.

TV/movie dates can be fun. Since you'll be in different time zones you might want to look into using DVR, or netflix, or just renting the same movie. Watch it and talk on the phone, or over an internet thing like skype or even just by IM. Make sure to start it at the same time and sync up, and you can have all your "omg gasp!" moments at the same time and talk about it as it's happening. Shared experience makes you feel more like you're actually together.

If you both have laptops with webcams, go out on a lunch date. Talk on the webcam, get a drink, order food. Go to 2 different restaurants that serve similar food - diners or Mexican or Chinese or what have you. Just make sure to find out ahead of time which places have wifi (preferably free wifi). This is super fun, but would probably get old if you did it more than once a month. Also it costs money and is less fun than actually going on a date in person.

Love letters are fun to send, and planned events and surprises are good too (eg. surprise webcam striptease when you had scheduled just plain webcam talking hanging out time)

When it comes to the sexual, find out what works for you. You can set aside that aspect of the relationship for a while if need be, or use any of the various communication methods... also, alterations of the other suggestions like the movie night - rent a porno or somethin' and watch it "together" on the phone/webcam.

Talk every day in some form.

Do things together - online games like MMOs, or watching synched hulu or netflix movies while also having a video conference on Skype.

Send each other little things as gifts to show you are thinking of the other person, and to make them feel appreciated and valued.

So... in essence, create shared situations that lead to social and emotional connection. It is the same thing that you do in meatspace, just without the meat.

Also be prepared for the odd effect of time spent together being both massively intense "Must.  Do.  Everything." but also tinged with sadness and an odd anticipation of time counting down "23 hours till I leave...22 hours till I leave...etc."

Second on the Skype movie date -- I like those.

Also: beware the pre-departure argument. It's easy enough to avoid, but somehow it seems to happen a lot to people who aren't QUITE in touch with what's going on in their hearts.