Exercising regularly usually works best for weight loss. You're riding on Saturdays and Sundays, but what are you doing on the other days of the week? Are you going for brisk walks? Weightlifting? Swimming?
If you are going to limit yourself to weekend rides, then you've really got to watch what you eat. Use a program like Fit Day or the Daily Plate to track what you currently eat, and learn where you can make changes. And add in some daily exercise, even if it is just 30 minutes of brisk walking every day.