Start to deckutter as soon as you are thinking seriously about moving.
Do a really thorough deep clean before agent comes to value, and again before photos - makes it much easier to keep clean quickly when viewings start.
Get a couple of plastic tubs to throw any bits and pieces into and bring in the car as you leave for a viewing (or going to work morning of a viewing). Just makes it easier to grab anything superfluous.
Put together your lists of utilities, all bank accounts, Insurance’s, all Dr’s, dentist, optician, schools etc - along with relevant ID or account or reference numbers. You will need to get in touch with them all to let them know you are moving. But you may also want to let potential buyers know what companies are available and their level of service (to answer their questions in viewings etc). I do a standard letter on my pc for letting all utilities know, and just change the ref at the top ?account number or whatever reference I have).
Start packing up non essentials as soon as you have agreed and know there’s a plan to move. The Christmas decorations, books, the “good” dinner service, the off-season clothes etc. Gives you a chance to declutter as you do it and less panic.
Label every single box. The room it is going into, and rough contents. I tend to put a loose ‘A4 sheet on top before I close it of the contents (saves rummaging through loads of boxes on arrival for the corkscrew...), and also put the contents into a “master notebook” with a separate page for each box. Some are detailed (exact list of kitchen tools) while others are more general (“history and garden reference books”).
But a packing tape dispenser and rolls and rolls of tape. At least 2 large black markers for room to go to and a box number or 1 word on contents, and 1 red to mark “fragile” boxes. (I tend to put an arrow pointing up on most as well).
Lots of trips to dump and charity shop as you pack.
Use lots of packing materials to protect anything fragile or valuable to you - tissue paper, bubble wrap, packing noodles, newspapers etc.
Don’t overload boxes - better to have half full and top stuffed with newspaper to keep things from moving than have it too heavy and more likely to be dropped.
If you think you want to take any garden plants, or internal fittings, either transplant to lots of possible for viewings or tell estate agent clearly to say it. And for futtings, consider changing before viewings to something cheaper and functional that you will leave there rather than changing later.