My top tips would be:
If you are going to be doing a lot of the cooking then you will be in a hot kitchen for hours. Bear that in mind when choosing your Christmas outfit - a big fluffy Christmas jumper is a no-no!
Don't do anything too poncy with the vegetables.
Make and freeze about 100 pigs in blankets in advance. They can be cooked from frozen on the day. You can never have too many PIBs.
Buy Aldi Christmas puddings. Don't bother with fancy ones with great lumps of fruit or weird flavours. Just do the traditional thing.
Nominate someone to be drinks monitor and serving all your guests. That includes them buying all the alcohol and soft drinks as well. They can also be nominated as Battery Purchaser, since they will be going to the shops anyway.
Nominate someone else (senior relative is ideal) to write down lists of who bought which present for the kids, so you have at least some idea of who they need to write thank-you letters to, and what for...
Nominate yet another person to be recycling monitor. There will be huge quantities of wrapping paper, bottles, cans, pulled crackers, empty boxes, you name it. The sheer quantity of rubbish generated on the big day is astounding.
The second you sit down with a plate of dessert in front of you is the moment your job ends. Make it abundantly clear to all present that your job is done, and the clearing of the table and the washing-up will take place and be done by others whilst you put your feet up for the rest of the day.