I went from 300 lbs in May 2008 to 200 lbs in May 2010. I am now down to 185 lbs or so and I have added a lot of muscle.
I started by really just watching what I was eating, and using Spark People to count caloric/nutrition intake. I basically just cut down to about 2/3rds my previous intake and stopped snacking as often. I was not eating unhealthy before, just too much 'good' food.
Add to that working out 5-6 days a week, cutting out soda and such, and restricting booze intake. I lost my first 30-40 lbs by just walking, light weights, and being more active in general.
Right now, I don't really watch calories anymore. My body is used to consuming what it should be, and I really need to stuff myself to go over my maintenance caloric intake. I need to make sure I am getting enough protein, though, as I am doing more lifting than cardio.