What are some good ways to stay calm in stressful situations?

I use the HeartMath heart focused breathing technique. I like it because you can do it with your eyes open, while you are among people and they won't even know you are doing it. It's more than just a breathing technique, it is also is a coherence-building technique. By focusing my attention in the area of my heart, I am able to adjust my heart-rhythm to become more coherent, and thus switch into a calm state.

I personally love the cortices brain balance technique for this! There are many videos out there demonstrating this and I have one I created myself.

Using your breath to bring your heart (HRV) into a coherent state is very effective. Coherence for adults is one complete breath every 10 seconds, or 5.5-6.0 breaths / minute, which causes a physiological response. Not to get to technical, but slowing your breath to the aforementioned causes a physiological mechanism that lowers blood pressure, induces a relaxed alpha state. It good as a sleep aid as well. It's important to lower your rate to that figure to gain maximum benefit.

I love what's already been suggested! Another tool I personally find helpful is Emotional Freedom Techniques (EFT) or Tapping.

Dr. Weil's 4-7-8 breath and Box Breathing (Mark Divine has a version that is easily accessible) are both great tools. I also like to remember that I cannot necessarily control an external situation—what I have control over is my reaction to it.

There is a meditation that helps protect and calm and it goes like this. Envision a large ring of Light above you, let it go all around you down to a bit under your feet and back up, 3 times envision that, do the same with a ring of light from the front to the back and back 3 times and from the right side all around you to the left and back 3 times. You can add and envision purple or blue light emitting from the inside and you breathing it in, blue and violet are very calming colors to work with stress. You can also wear blue or violet colored glasses. You can get therapeutic colored glasses via amazon. All the best.