Another good one that has really helped me lean out and maintain mass has been explosive Burpees. Start in a standing position, squat down quickly then drive your legs out and put your hands into push up position. Do a full push up, bring both knees up to your chest quickly, from there jump as high as you can. As soon as you land go right back to your next rep, and go for 20 reps. Do each rep as fast and as explosive as you can. 1 min. rest in between sets, and drop 1 rep every set until you hit 10 reps. Finish out your 10 rep set and your done. I was in pretty good cardio shape when I started doing these and they kicked my a$$ the first few times. Awesome fat burner in a short period of time. Plus, you don't need nice weather or a track to do them. Any time, any where.