When my daughter has a meltdown, or I have a meltdown, or when we have a meltdown together, I have always used breathing as a solution for calming us down.  IT WORKS!  Please do not doubt this technique.  Just give it a shot.  This is for children and adults alike. It is a great distraction for us bot, a distraction away from the moment that we are stressed about, angry at, or just plain frustrated with.  It slows us down and gets us our focus back to a moment of clear thinking. Without being stressed to begin with, this is a fantastic way to just relax.  This technique is about controlling your breathing.  I have done a more countdown-simple type of breathing technique with my daughter. The first breathing technique mentioned may be better for adults.  It's totally up to you.   
 
*Sit up straight wherever you are and start to focus on your breathing, listen to the sound of your breath and start to take notice of your breathing patterns.  Count the length of each inhalation and exhalation.  The goal is to see if you can make both the same length.  Don't be forceful.  Give it time to even out gradually.  Finally at the end of each exhalation, try to hold your breath, building up to the same length of time as the breaths: for example breathe in four, breathe out four, hold for four.  Practice until it feels easy, then return to regular breathing.
 
*With children, during a moment of injury or a crying fit that won't end, before bed time, or a pure meltdown...we sit together and I say "breathe in...feel your belly rise,"   "breathe out."  We sometimes imagine colors that make us feel better to breathe in, and colors that make us feel bad to breathe out. Sometimes we take the time to count each breath.  It may sound impossible to get children focused enough, but it has really worked for me.  Starting at a young age is even better.
 
I hope the best for you and these techniques!