Having gone through this 3 weeks ago, here's my advice:
Pack enough stuff for you and your family for 3 days - clothes (remember any clubs etc children need to get to), shoes/trainers, coats, toiletries, and keep this in the car so it doesn't get swept up in their packing.
Keep out some cleaning stuff so you can clean rooms when they're done.
Keep enough mugs/kettle/tea/sugar etc out (they'll make sure you do this!).
Keep them happy - keep supplying biscuits, doughnuts, sandwiches (ours worked through lunch and dinner), fizzy drinks etc.
Ours underestimated how long it would take, so on completion day, they didn't even start loading onto the lorry until about 3pm. We were eventually out by 7, and by the end, stuff was just getting shovelled into boxes, neighbours were helping etc. The new owners were seriously NOT happy (although thankfully not trying to move in the same day). And then we couldn't get unpacked that night, they had to come back the next day, and for a while I wasn't sure we'd be able to sleep in the new house, I was ready to decamp to a hotel.
We thought the fridge was ready to be loaded, so emptied the (small thankfully) contents into a cool bag. In the end, we didn't get a fridge back for about 30 hours, so try to make sure you've got NOTHING in the fridge - we had to throw the lot out. And don't let your DH go out and buy everyone microwave meals on the the night of moving in when there's no prospect of seeing your microwave for another 24 hours - more food that went in the bin.
But overall, it's a great service, and at the end of the day, you'll be in your new home, so it's all good! Good luck and congratulations!