The most relaxing yoga breath ever...

6:9:3 - There is a whole science behind using ratios in pranayama practice... different ratios of inhale, exhale and pause can produce dramatically different effects.  If you are interested in diving into it, I suggest finding resources from TKV Desikachar, AJ Mohan and Gary Kraftsow - they all speak to this in their books.  

I learned this particular ratio years back from Gary Kraftsow and I turn to it whenever I am in need of some serious relaxation... You can use your favorite pranayam or just breath as you institute the breathing ratios.  

(My pranayam preference for this ratio is to do a 3 part inhale from the bottom up and 3 part exhale from the top down in a variation of the Dirgha Breath that I learned at Kripalu.  This most closely matches my natural breath patterns (making it more naturally relaxing) but the awareness helps ensure I build my lung capacity and control.  I have also used this ratio to great result with Anulom/Vilom) 

In a comfortable position:

INHALE 6
EXHALE  6 
REPEAT 6x

INHALE 6
EXHALE 9
REPEAT 6x

INHALE 6
EXHALE 9
PAUSE 3
REPEAT 6

After your last pause, release the count as you continue to breath.
After the next exhale, release the conscious pause.
After the next exhale, release all conscious control of your breath and just be.

It is the systematic, progressive slowing of the breath that helps to slow the mind and bring the nervous system into parasympathetic mode (rest and digest more :-), thus creating the most relaxing breath ever.  *Note: It may not be wise to operate large machinery after this practice!