Well, luck determines your success many times. Sometimes I've gone into the quickenings with all 3 characters at 3 mist charges, and after the first person's quickening I couldn't get either of the other 2 characters' quickenings to show up despite spamming R2. Other times, and actually my best chain(19) occured this way, you can go in with only one mist charge on one person and get lucky and get a huge chain. I think certain boss fights are especially prone to making chains harder/easier to get. My chain 19 came out of desparation because I was about to die and spammed the quickening to get in one last shot. After Vaan's initial Red Spiral, Ashe kicked off the craziness with 3 Maelstrom's Bolts in a row, after which 1-2 of Vaan, Baltheir, or Ashe's quickenings were available immediately after the previous quickening. Only once did I have to even R2 to get a Mist Charge to show up, to get 12-13. I think there were 6 lvl 3's 5 lvl 2's and 8 lvl 1's, so they busted out Black Hole and saved my butt. So there's definately an element of luck involved. Other than that it's all about reflexes. Don't waste time early on because the faster you get your commands in, the more time you'll have for the next quickening.