This isn't done by sending gifts in the mail (although she does do that, but not to the point that the boys expect it). We periodically Skype (the times we don't are all my fault, I forget about the time difference until it is too late to call), and she comes up with creative ways to engage them.
Before Christmas she sent them each a story with a cliff hanger. They boys had to write the next part of the story and send it back to her. We are getting ready to send part 4 of the stories to Ontario. It takes us awhile to get all 3 stories written, it is mostly Darien that makes the process a bit longer. Grayden dictates his to me and Connor writes his own, but Darien (who is in grade 2) is still learning to sound words out and gets frustrated easily. I don't push it, when he is done we take a break, I don't want him to hate the process.
As they write them I have been typing them out in case of any lost snail mail letters. It would be easier to e-mail them to her but that takes away from the excitement of getting your very own letter in the mailbox.
It makes me want to have a pen pal of my own!