Always make a point to use natural light to the best of your ability. Place areas of most use (ie. dining or living areas) in the spaces that get the most light. Avoid crowding items into a space as this can make it feel smaller and darker. Better to choose a few high quality items and give them room to breathe.
Before you decide on a new paint colour for your room, be sure to paint swatches of all your colour options on every wall. This will allow you to fully analyse how it will look under all of the different patterns of lighting from both indoors and outdoors (don’t forget to check how it looks in both day and night!)
The best designers are those who can find compromise between their artistic vision and the practical needs of their client. For example: an expensive white leather sofa would be a very poor choice for a client with kids and a dog, no matter how well it fits in the colour scheme! Something durable and easy to clean might be a better option for a family.