I agree with face timing, I also have my MD or DB send a short video of themselves that I can play throughout the day. So it's just a little message of them saying hello and waving and blowing kisses and saying "I love you," that sort of thing.
We also have special traditions for when MB or DB are not in town that we can't do when they are. For example, my MB is really allergic to Keewee, so we pretty much never have it in the house, but when she is out of town, I always stop at the grocery store and buy some Keewee naked juice for NK. It's not a big deal, but just something that reminds her that sometimes it can be fun to have a different caregiver.
We also talk about the things that their parents have contributed to them, even if they are not around. This is less someone to do specifically when they are traveling, and more of just a general habit. So when they are putting on their pajamas at night, I will tell them how cute they look in their pajamas and point out that mommy bought those for them. Then I will add "she sure loves you a lot, to take such good care of you and make sure you have such nice jammies." Or, when I am making a snack, I will say "Look! Daddy remembered this is your favorite kind! He sure loves you a lot to make sure you have just what you want for snacks.". Again, this is something that we do just on an ongoing basis, but I think it helps them to remember that their parents are always aware of and providing care for them, even if they are not physically present right now.