Dry your brushes upside down. 

Layers, layers and more layers 

Be careful when using black to darken yellow or ochre.

Are your lights light enough and your darks dark enough?

It's much easier to make something darker than to make it lighter.

The further away it is the paler and more pastel coloured it becomes.

Never leave your paintbrush in the water. 

Always check the direction of your brush/pencil strokes. 

Every art piece has an ugly phase, or two.

Acrylic is very forgiving, you can always paint over it if you don't like it. 

Be careful of painting over something that's only partially dry.

Straight black is boring. Mix it up!

Use the right tools for the job.

It's often easier to paint the background first.