I thought I'd write a post about High Intensity Interval Training (HIIT), which in my opinion is absolutely the best way to use cardio to burn fat while preserving muscle.
Using running as an example, you run for 30secs, then walk for 30secs, alternating for 15-20mins, depending on your fitness. The great thing is you don't need to be that fit, and its a great way to build up fitness to get into continuous running, and burns more calories than just walking. Its also funner than straight running or walking because its challenging, and you're changing what you're doing every 30secs. If you're really fit you can alternate sprinting/jogging, on an exercise bike or x-trainer alternate 100RPM and 60RPM (and adjust resistance level). As you get fitter you can increase, up to 2mins "fast", but I wouldn't ever go lower than 30secs "slow", so your heart-rate can come down and ATP-PC restored. If you're quite unfit you can use longer rests as well (30secs fast, 1min slow). Lots of options!
There are plenty of studies on the net but here is one that shows HIIT is far more effective for fat loss than continuous endurance training (ET)
The reasons why it is so effective, as opposed to continuous cardio, is that as the heart-rate elevates to over 60% of your max, your body can't turn fat into energy quick enough, so it starts to use muscle. However, if you exercise at 60% or below, you're not exercising very hard, and not burning many calories.
HIIT gives you time inbetween bouts to recover, lets your heart-rate drop, and time to break down glycogen and fat stores to provide more energy, so your body uses far less muscle. Preserving muscle is the major key to conistent weight loss (otherwise your metabolism slows down and weight loss stops).
Also a major factor in burning fat is post-exercise calories burnt while recovering, repairing damage etc. HIIT seems to activate more post-exercise calories than continuous training.