I have to agree with Stackt on this. Keeping the heart rate in the fat burning rate is the trick.

It's boring but if you are walking/rowing/cycling at a rate that seems piss easy then you are probably in the right range. Do this and you will burn fat calories and no muscle at all. There is a scientific formula for the fat loss rates but I can't find it now. You won't be burning as many calories as other forms of cardio but it will be fat you are burning.

Be warned: it is tedious, but if you stick at it and maybe split your sessions up during the day it will work. Also, because you won't be sweating there's no need to shower etc afterwards. Not unless you've just finished a workout!