Eat healthy(diet soda is key!)...
Actually, if you're talking HEALTH... soda at all is very BAD. Especially Diet soda. It has artificial sweeteners in it which are part of the MSG family (MSG kills brain cells by tricking your brain into thinking that something tastes one way when it doesn't).
You'll be surprised to see how many pound one drops when they stop drinking soda, eating pizza, eating ANY fast food, and eating processed foods (ya I know that's impossible for some people, those evil grocery stores). All natural is the way to go.
I lost about 20lbs pretty quickly (in a month I believe) way back a long time ago... by eating (and yes, I way healthy while doing this, though I calculated it for my personal BMI, this might not be so healthy for everyone) - I took too many nutrition classes...
Breakfast: small bowl of cottage cheese with a peach OR a bowl of cherries
Lunch: Peanut butter and Jelly Sandwich
Dinner: Hardcore Soups and Stews (most of the time), once a week a nice plate of food with chicken or steak with mashed potatoes and veggies.