As well as light walls make sure your flooring and large pieces of furniture are nice light colours. Masses of dark wood furniture will make a room feel darker. The same goes for e.g. banisters, fireplaces etc. Maybe light rugs if you can't replace the flooring? The same goes for dark works of art on the wall.
Trim trees/hedges etc outside the windows.
Are your window coverings blocking the light? For example, make sure your curtains sit almost totally on the wall when they're drawn so that they aren't blocking the window during the day.
Are the ceilings light? Are their eg dark beams you could paint?
Add lots of lamps, make sure your lightbulbs are bright and a nice 'tone' of white - I like warm bulbs but I know some people think cooler bulbs simulate daylight better. Use your light walls/ceilings to reflect light by having up-lighters or lamps against the wall. Metallic lamps will reflect the light increasing how much light they add.
Convert windows into french doors if possible. (With a Juliet balcony if upstairs).
Consider a skylight where possible. One at the top of the stairs illuminates the whole stairwell and lights the house from the interior. Having a bright hall adds light to every room in the house.
Have a glass front or black door?