How would you keep alive a relationship in which one person was always traveling?

It really depends a lot on the people in the realitionship. With skype and factime it can made easy to contact one another. With different time zones it may be hard but before a trip try to plan a specifc time to call/text/facetime.

Personally, I would make sure to always keep in contact, even if you could not see that person face to face. I would use common day technology like factime, messenger, etc. If two really want to make a relationship alive and working, they would put in the effort needed. Also, the person always traveling would need to eventually make at least a tiny bit of time to actually come and see the other, even if that be two years down the road from now.