I'm in good physical health, exercise regularly and having been training capoeira for about a year. While technique is certainly a part of it, I've still been unable to kick more than waist high with an Armada or Queixada (dictionary of terms). I feel like the tension during a kick is in my waist and hamstrings. What stretches or other techniques can I do on my off days to help get my kicks higher?
A good place to start looking is ExRx's list of stretches per muscle. Generally, whenever you feel that a muscle is not flexible enough for your goals (what you described as "tension"), you should try to make that muscle more flexible.
For the kicks you mentioned, it seems to me that hip adductors and hamstrings would be the muscles involved, but I never executed those particular movements so I'm not sure. The link I gave has stretches for those two muscles (and many more).