1) Neutralize with paint.
2) Remove dated wallpapers or paint over them.
3) Scrub spotlessly clean and de-clutter/edit.
4) Add some contemporary/modern artwork.
5) Beware of lots of dark wood. Painting wood or lightening it can be helpful.
6) Replace outdated hardware.
7) Refresh bathrooms.
8) New appliances can help a kitchen feel newer. Many buyers open fridges!