Maybe send a newsletter to everyone informing/explaining to them the "limited" availability over the christmas period... and if they need extra work it needs to be pre schedualed. I would also have a cut off date (eg 5 december) for any extra's that can be prebooked if your already busy.
Might be a bit late this year to be effective but something to put in the calender for next November!
Overall, being busy is a nice problem to have
Simmo.