-If you're visiting friends or family, don't monopolize one particular member of the household and leave everyone else out.
-If you're visiting a family that has a much-loved dog, don't visit all day and demand that the dog be crated/confined to another room with one person for the entire time. If you're allergic, that's one thing, but if you just "don't like dogs," that's another.
-If the family normally does a "round robin" gift opening, don't snatch gifts out of other people's hands so you can see what they are, before THEY even have a chance to, and then blame them for being "selfish" for not showing you right away. This goes double if those people are children.
-If the children of the family show appreciation for gifts other people have given them, this doesn't automatically mean that they're "ungrateful" about your gift to them. Also, bear in mind that, to a five-year-old, a ten-dollar plastic sled is going to look more appealing than a $25 cheque, despite the discrepancy in monetary value--to a young child, a cheque is just a piece of paper with writing on it.
-If you particularly dislike a certain member of the household, either decline the invitation, or be nice to that person anyway. After all, much as you dislike this person, he or she invited you over for Christmas anyway, so that counts for something.
-If you're a member of the "host family," don't insist on inviting a toxic person over for Christmas just because "they're faaaaamily!!!" I know it sounds heartless, but sharing DNA doesn't give you a free pass to treat people badly.