Consider removing curtains, reducing their size or avoiding heavy materials all together. This will increase the feeling of airiness in a room whilst opening up the view to outside.
Define clear walkways and don’t have furniture blocking the view into or across a room. Select taller pieces of furniture to utilise the available height and consider vertical panelling for your walls. - MitchellMc
Short of knocking a wall down… white walls, pastel/flat colours, and mirrors will help. Furniture helps but should complement the limited space you’re working with. A lack of anything makes it look small but if the room looks like it has purpose without being crowded you'll be onto a winner - ProjectPete
Decluttering is a focus at present. So many people are raving about Marie Kondo at the moment so I feel very much on trend. - Alicia