We had one deployment with a preschooler and are getting ready for another, now with 2 young ones. Many things come to mind when I think of successes such as skype (since my second child was an infant during the deployment and then a walking toddler upon the return), phone calls, letters and packages, lots of family visits and time with friends, etc. But two things that stand out to me are: the departure gift....this is a small gift from daddy at home that my preschooler received after leaving daddy at the airport. It was a fun surprise and helped to bring joy on a very difficult day. And the second are the "daddy dolls", www.hugahero.com. My son loved his and it came with a recorder inside on which my husband left a touching message for him. It played every time the pillow was squeezed. It's amazing how the little things make a difference.
We too used the daddy dolls for my young twins when my husband deployed to Afghanistan again last Spring. They loved them and even my twin boy is still attached to it. Taking it to bed every night, etc. They are so easily washable too, love them! I ordered my older one's pillow cases with pictures of him and their dad doing things together on it from the same site. Made for lot's of smiles and good memories before turning in for the night.
Our little family is on the tail end of my husband's year long deployment to Iraq, LORD willing. I must say that it has been both harder and easier than I though, and that having low expectations of what things might look like with young children is very important. I got a Daddy doll (www.hugahero.com), like the other responders here, and that has been a great encouragement for my 3 year old. We also had some videos of Daddy with the children which my 3 y.o loves to watch. I have 2 young children and the easiest thing for us was phone calls, whenever they were. I couldn't seem to make a regular video date to skype work for us, which was somewhat disappointing, but I kept reminding myself that it was counterproductive to be stressed out by trying to be encouraged by a video chat!