Avoid too much ceiling lights - less is better. A lot of homes for example put way too many downlights.

Make sure you don't forget dimmers. They are fantastic for mood and entertaining and very expensive to install later on.

A huge mistake is to put lights in the wrong areas, for example in the dining room you don't want bright lights shining into or above people's faces.

A mistake I see everwhere is to put a hige light like a chandelier either in a small room, or over a small table. Really looks nasty!

When you buy the lights don't depend on "gut feel". Take measurements into the shops.

When you install pendant lights or chandeliers, it can be extremely easy to accidentally make the bottom of the light too low. Make sure it's not.

Don't go for too many feature walls that are too dark, as it will destroy the lighting you put in.

A very common mistake is to put cool white or daylight white lights / globes instead of warm white. Homes should be WARM white.

Light switches that are too high or too low, or positioned in the wrong spot. Amazing how many electricians do this.

Make sure you light up small or out of the way areas. They are very easy to neglect.

A huge mistake is to not make your kitchen bright enough, and also in the kitchen you need to have light above each task area.

If you put in downlights, make sure they are in a nice neat array, and not too close. 

Avoid the mistake of installing modern lights inside older homes. Looks weird and irritating.

Forgetting dimmers is a common bad mistake.

Lots of homes put in really bad lighting in the bathrooms. Just because it is the bathrooms does not mean you should forget about it.

A common mistake is to have only one light source in a room, such as a pendant light or an oyster. Put in a number of lights to create a much better atmosphere.

A common mistake is forgetting about natural light.

A mistake a lot of interior designer see quite often is creating harsh shadows.

If you use downlights, make sure they have large beam angles - small beam angle can create pools of bright lights surrounded by shadows.