Everyone's talking about adding things to the house, but that could simply add clutter.
Walls, ceilings and floors need to be clean. No stains, no obvious paint marks, well maintained wallpaper, smooth walls (if painted) and clean carpets, rugs or floorboards.
Whatever you have in your house, make it the best quality you can afford.
Charity shops sell good, solid furniture - brown vintage or you could 'shabby' it to match your decor.
Soft furnishings go the same way - the best you can afford, clean and well maintained - cushions not threadbare, curtains or blinds well hung and running freely, throws cleaned and fresh.
Think about layout. If some bits of furniture can only fit a certain way, then you've got to work around them.
You've got to try to make spaces 'flow', with unimpeded access to doors and windows - less claustrophobic and safer in emergencies.
The finish on skirting boards, pin holes in walls, old hooks, the boxing in of pipes and the general 'finish' of carpentry, painting, plumbing or other DIY jobs need to be considered.
If your house was a very expensive hotel room, would you pay 'top dollar' to live there.
If not, why not?