I find it best to expel all the days' inhaled nonsense. Take several deep-as-you-can breaths, then fully exhale forcing every minute pocket of your lungs to release the air. Repeat this several times and when you start yawning, you know you've done it correctly because your body will be rejuvenated with fresh oxygen almost tricking your body into being tired. I find I sleep much more soundly if this is the first part of a pre-sleep meditation.