The three-part breath
Most of my preferred practices help me tap into the parasympathetic nervous system. And the key to kicking-in that rest-and-digest? The breath.
Easier said than done, I know. I am a reformed reverse-breather.
Learning to breathe into different parts of my body has been life-changing.
The three-part breath helps us calm and ground ourselves. As you approach this breath, don't be an over-achiever about it--avoid forcing your lungs to max capacity. Aim for a slow, smooth, and soft wave of breath on each inhale and exhale.
How to practice the three-part breath:

Get comfy. Make sure you spine is long.
Close lips.
Hands on belly.
Inhale through nose, belly expands
Exhale through nose.
Hands on ribs.
Inhale through nose, ribs expand.
Exhale through nose.
Hands on chest.
Inhale through nose, chest expands.
Exhale through nose.
Relax hands at sides.
Inhale through nose, directing the breath into the belly, then ribs, then chest. Feel the wave of your breath.
Exhale through nose.
You can do this anywhere: stuck in traffic, while making dinner, or before you go to bed. Keep this breath in your back pocket for anytime you're feeling a bit out of sorts.