2. Change the way you exercise - All aerobic exercise will burn away some muscle. It's a balancing act trying to keep yourself in good aerobic shape while trying to maintain muscle mass. I try to reduce the amount of time spent exercising (don't do more than two hours a day) but ratchet up the intensity of the exersise.
When jogging for example, sprint for 30 seconds, jog for a minute, sprint for 30 seconds, then jog for a minute, etc. This seems to encourage the muscles to maintain some mass while still getting great aerobic exercise.
Different types of weight lifting routines are supposed to be good for building mass. I actually try to reduce my muscle mass while increasing my functional strength (that way I'm strong for my weight class), I tend to do fewer, quicker repetitions. If you just want to bulk up do your weight lifting more slowly and with more repetitions. Rather than pushing it up, bringing it down and saying 'One!', take 5 to 10 seconds per bench press.