It's easier if you're going house to house (we've done house to storage as well which is a pain).
You need to have kettle, tea, coffee, sugar (!!!) Milk and biscuits out and ready to use. The packers will pack them last and unpack them first. Be generous with drinks and food - I've never got away with a single packet of biscuits.
Precious things (paperwork but also jewellery, cash and musical instruments) I move myself - check your house contents and movers contact to see what they're covered/not covered for.
Arrange parking for them if they've not done it.
Get your kids out of the house. Anywhere is better than home. My DS1 was utterly traumatised the first time he was aware of moving house because we went back to the house and nothing was there (move to storage). Never again. We've begged help from anyone willing to avoid it all the other moves.
Pack day sacks and overnight bags for each child - so it doesn't matter if the only things unpacked in your new house is the bed, you can get them down and keep unpacking.
The movers might not be obliged by contract to reassemble beds/wardrobes etc. Ask very nicely and be prepared to tip in cash if they do it for you.
You might want to check that your movers are using their own people rather than agency. Things tend to walk and hey broken less when it's own-company movers.
If you don't have time to declutter I wouldn't worry. Several times we've moved and as I've unpacked I've thrown stuff out (thirty black bags type throwing out). It would have been cheaper to do it first but life isn't always like that.
Have wee plastic bags (for nuts, bolts etc from deconstructed furniture) to hand. Have spare marker pens so every box gets written on.
Have a spare toilet roll in the loo - we've had to go out to buy it when the upstairs cupboard was already packed and the inevitable happened.
Make sure you move anything that's included in the sale or that you're leaving for any other reason out of the way. They pack in a whirlwind so if it's not away or signposted to them before they start you won't see it again until it's unpacked.
Make sure you update your house contents insurance to cover the move and the new address from the relevant dates.
Good luck!