drinking water is far better than diet soda, thats definitely true. Herbal teas like Brother FreakoffaLeash recommends is good too. But there is also such a thing as drinking too much water or tea so keep that in mind. With any good balanced diet especialy one to lose weight you need portion control and exercise.
Its better to do a lifestyle change than just a 'diet' change. Its also good to lose weight slower than losing it fast. Be aware of what is in food and know that many 'diet' products may not be quite what they seem. 'no sugar added' products don't always mean low calorie. Often they just add other kinds of 'sugars' instead of your typical 'cane sugar' such as glucose or fructose. Pretty much anything that ends in 'ose' is a sugar.
Its fine to treat yourself for reaching a weekly or even monthly goal but instead of going out for that high fat/calorie combo meal at your favorite fast food restaurant, try something more balanced. A couple of low calorie cookies or an individually wrapped 'mini' chocolate bar works instead.
Oh speaking of fast food places, be careful with salads from them, often the additional things added to the salad can realy hike up the calories and fat (and salt). These days all restaurants have their nutrition list of their foods on their websites. Its good to familiarize yourself with them and see how most really don't have anything super low calorie. Sometimes the lowest calorie stuff is just the coffee or tea.