This exercise, like many to follow, focuses on the breath, and is perfect if you have only 5 to 10 minutes. Close your eyes, get comfortable, and breathe in deeply through your nose. When you do, imagine you're breathing in fresh, clean air, that's bringing lightness and new energy into your body.
And as you exhale, imagine toxins or negative thoughts and emotions leaving the body.
It can help to assign colours to each breath. When I inhale, I like to imagine I'm breathing in clean, light blue air. And when I exhale, I use the colour green and imagine I'm breathing out hot, sticky, polluted air.
The reason I like this exercise is that enough is going on to keep your mind occupied. So it is less prone to wandering off, and you can focus for longer on your breath.
Source: As a burned out banker I attended a class on meditation in Canary Wharf, London's financial district. Even though this technique has been around for centuries, the monk who led that session introduced me to it. To that person, I say both thanks, and sorry I couldn't properly give you the credit you deserve.