Christmas has always been a really special time for me.  It's changed over the years - not always for the better - but I strive to keep that specialness in it no matters what.  Now I have kids, I feel even more strongly that Christmas is a special time

Make traditions
Most kids - including mine - seem to love to know what's going to happen next.  One of the easiest ways to do this is to set up traditions.  Doing things the same way each year.  Putting up the tree the same time.  Putting out stockings the evening before.  The kids know what's coming and feel they have more control over things than if the situation is constantly changing from year to year.

That's also one of the things I missed most about my childhood.  Our family had our own traditions when my parents were together, but after they separated, none of the old traditions stayed and each year was different.  It made Christmas feel so much less special and contributed to a lack of feeling like a family.  Those traditions were part of our identity as a family so to take them away meant taking away our family.  I hope by giving them back to my kids they can feel like part of a family, even if that family is spread over 2 households.

Communicate with your ex
Not always easy.  But if you can, find out what they want the kids to know about Christmas, e.g. is Santa real, do you tell the nativity story, do you exchange presents, do you celebrate some other festival at this time?  If you can, present a consistent front on these basic ideas behind Christmas, or if you have different ideas which you cannot resolve, try to at least present your own ideas in a respectful manner to the kids without judging your ex's.

Use a neutral third person
One of the things I've found has really helped with our kids, we still attend events together for our kids (e.g. school concerts) and we are also spending Christmas morning together with the kids.  BUT we don't  do it alone.  My mum - who gets along with us both - will be there, which helps defuse the situation and also means it helps keep our focus off each other and on to the kids where it belongs.  It also takes the pressure off the kids because they don't have to choose between Mum and Dad on special occasions - they still get us both.

Take lots of photos
I need to tip my hat at Jim Rohn for this one (you can check him out at www.jimrohn.com).  He recommends taking lots of photos throughout your life to help record your memories when your own mind fails a little.  I think this is even more important for kids whose parents are separated.  Its part of helping build their family history (see article by karenalonge on "Ten Strategies for Co-parenting with an Uncooperative Ex") and helping them feel that they belong to something - even if their parents don't belong together anymore.

Give your ex a gift
This is part of being a bigger person and demonstrating to your kids how to be loving and forgiving.  My kids are still too little to buy gifts themselves, so I buy gifts for them to give to their Dad at Christmas time, and I also buy a gift from me to him.  Again its part of encouraging that feeling of belonging, but also its hard for someone to be angry and hostile while accepting a gift.  I might add, though.  Be prepared that this will be a one way thing.  You may not get something back.  You may not get a thank you.  But this isn't about your ex.  This is about who you are and what sort of person you want to role model to your kids.

Do something for yourself
Chances are, that if you're sharing Christmas with your ex, you're going to be giving out a lot of yourself when he/she's around. This is not easy and you're going to need to recharge.  I get time after lunch when my ex takes the kids to spend time with his family, but it could easily be the next day or so later.  Even if you have to get a babysitter, get yourself some time alone when the festivities are over to recover.  This is a very emotional and stressful time which is made more so by being confronted with your ex, even if only for changeover, and you need to let  yourself reenergise so you can be your wonderful, loving self again. 

That's all my ideas so far, I'll let you know if I come up with some more.