I simply adore koicha!!!!
I've been studying tea ceremony quite intensively for the last 18 months now, and I've reached the stage of making koicha now (I've made koicha both in the 'ro' and 'furo' seasons).
As someone suggested, it does require a lot of practice, and patience. I find that hot water REALLY does make the difference between having a beautifully made bowl of koicha, as well as the ratio of hot water to maccha; otherwise it will turn out very gluggy and almost not fit to drink!!
Personally for me, if I'm preparing a bowl of maccha for 3 people, (san-me-sama, as we'd say it in the chanoyu), I prepare 10 heaped scoops of maccha, and using a hishaku (bamboo ladle), I tend to pour in 3/4 of a scoop of water first, whisk the maccha 150 times (50 per person), then I pour another 1/2 scoop of water, whisk again until all the tea has completed been taken up by the water. It is definitely best served piping hot!!!
Hope that hasn't confused the hell out of you all! Just thought I'd share my experience of koicha.