My single biggest tip would be to cut down on all the extended family activities. This may not be a problem for some people, but for those with young children and large families, it can be quite exasperating at times.
My husband has a very large family spread throughout the state. In the earlier years of our marriage, we would often end up toting two little ones around on car trips that were two hours or longer each way on Christmas Eve or Christmas Day in order to visit everyone and attend all the different family gatherings. We would do this for multiple family gatherings, all within 36 hours or so of one another.
We finally decided that this was a silly way to spend Christmas (which indeed it was!) From that point on, we've agreed to one family event with his side of the family and one with mine. They can be on Christmas Eve or Christmas Day, but must be in our city for us to attend. We both feel that this is more than reasonable and still gives us some time to spend Christmas just as our own little family at home too.
I would suggest that everyone set their own limits on these types of gatherings during the holidays. It doesn't have to be done in a mean way -- we love all our family very much and love to see people during the holidays -- we just needed a little breathing room for ourselves, too. Of course, everyone's tolerance for these types of things differs, so what works for us might not work for others, but it was one the major keys that allowed us to truly start relaxing and enjoying our holidays.