Measure the water exactly - which must be ice cold from the fridge. I use a hand blender for at least a minute - probably for about 2 minutes to make the mousse lovely and light and fluffy.
That's how I do it, and it always turns out really tasty!!
make sure the water is ice cold before blending. Put the mix a mousse in the sachet and mix up with a spoon. Remove water from freezer/fridge and blend either in 2 or 3 goes for a good few minutes. Then return to freezer for around 10 minutes.
This is how I make it and it is delicious! Think it is something you have to perfect though!