So disappointing and assuming you probably have a disappointed buyer to deal with as well! If I was you (and I have been!) I would:
Agree with pp's that the first step is a big de-clutter, I'd invest in getting a skip as it makes getting rid of things so much more straightforward. Donate any old toys/clothes etc that are in good condition then get rid of everything else that you don't need. Try to be as ruthless as possible as you'll only be paying someone to move it in 6 months time anyway.
You don't say if you have kids but my 3 DC are complete hoarders (eg. small bits of cut up paper seem to hold massive sentimental value) so if you can do their rooms when they're out that's always easiest. Or if it can't be helped a compromise where things "go into the loft" for a few months then get binned when they've forgotten about them.
The next stages I'd do room by room rather than all in one go...en once you've done that assess what storage you're lacking so that you can feel like everything has a place and it makes tidying up much easier and makes everything feel more spacious. Cabin/storage/high sleeper (depending on age) beds can make kids rooms a lot easier to manage and give more floor space. You can also take them with you to your next house.
I'm assuming you might not want to decorate but if you think you might be up for painting it's definitely the cheapest/easiest way to give a room a refresh. If not can you revamp any of your furniture, I've seen some amazing transformations of basic IKEA stuff, there was a programme on Channel 4 Recently, I think it was called something like The Great Interiors Transformation or something with Emma Willis, that had lots of good ideas.
If you don't want to revamp anything (or once you have) then take a look at your furniture's position , is it in the best positions for how you use the room or more importantly how you'd like to use the room? Even if you can't envisage it (I'm not always great at this) you can try moving it around to different positions. It can make a massive difference.
Finally soft furnishings and finishing touches, I always find a new set of cushions and a rug can really make a difference. There's loads of great ones for reasonable prices, I like H&M Home and M&S for cushions and George at Asda do very reasonable priced rugs.