I find that the best way to drop pounds by exercise is by doing cardio that uses more than just your legs, ie swimming. Although, if you don't have the endurance to swim for 45 minutes straight, I wouldn't do it. It takes lungs of gold to swim hard for 45 minutes.

I heard rowing is pretty intents too, but I haven't tried it.

Also, as stated above, aviod monotony. Do a different cardio routine for each day of the week that you do cardio. You may also want to do cardio and weightlifting on different days, because they counteract eachother a little bit.