Every couple has something in particular that tends to make situations more tense while traveling. For example, if you and your partner are terrible with directions, always have a map or GPS on hand instead of driving each other crazy trying to navigate on your own.
Plan out your itinerary before you set off on your trip, getting a good idea of where all the attractions are in relation to one another so you don't end up spending hours getting lost. Similarly, if your significant other is a planner and gets stressed out when tickets and hotels aren't booked far ahead of time, make sure that everything is taken care of early.