I read this advice in a magazine, and used it when I went out of town for a week. It helped quite a lot. Instead of getting gifts to give your kids when you get home, get or make them each a gift box only to be opened after you leave. I went to the dollar store, and picked up a large gift bag for each of my kids, then I filled each bag with little goddies that they don't normally have around the house, (bubbles, glow sticks, etc.) for them to play with while I was gone. I also wrapped a few, and labeled them, "do not open until Wednesday" or thursday, or whatever... that way they got a new one while I was gone as well. I didn't make any of them expensive, just things that take up some time, and that I know my kids enjoy doing.
Also, the more emotional you are when you leave, the more emotional they will become. Be excited for them, tell them that it's their special time with daddy, or maybe have them help plan a night doing something with just him (like maybe watching a movie and eating in front of the tv)...
The last piece of advice, and this sounds cruel... but don't call them while you are gone. All this does is remind your kids that you aren't there, and when they talk to you, it will make them (AND YOU) more homesick for them. Plus, you'll hear about everything that isn't going well, (if there is any) and your guilt will skyrocket!!
Hope this helps!!