Buteyko method is great for so much more than just asthma, as it resets the autonomic nervous system. I began to sleep better in very little time. The rest is slow going for me, though, but I know it's helping.
A very simple exercise is to cup your hands over your nose to reduce air flow, for a few minutes, many times a day. You should feel a little air hunger. Open up a crack if you start to feel too much air hunger. There are many ways to accomplish the same thing, reduced breathing, so you find the ones that work best for you.