We work really hard to avoid this dynamic, although it can be tricky! I find the easiest way to achieve that is that every time our older child expresses a verbal preference for one parent, that person will redirect his love to both parents.
I'm amazed at how well it works! And also at how hard it is to keep LO from having a preference... a small part of me does want to be his favorite, after all. But a much larger part of me wants both kids to love both parents a whole lot... and thank goodness for that.
Do you compete at all with your SO for your LO's love and affection?