I take seroquel as needed. Maybe once or twice a week. I have no weight gain from this. Although I only take 50 mg.
My only problem now is that the day after I take the seroquel I am tired in the morning maybe even a little agitated - but extremely energetic in the evening and I end up having to take a klonopin just to calm me down.
If you continue to pack on the pounds talk to your pdoc and I think there are things that can counteract that effect.
Unless you have certain dietary restrictions in the mean time, I would recommend looking into eating fewer carbohydrates. I would cut out all refined carbohydrates and just eat protein and veggies / fruit. This works extremely well for me. I will not touch anything made from white flour, high fructose corn syrup, or hydrogenated soybean oils etc... It might be a good idea to start walking as well.
Maybe you can look into Atkins, or the South Beach Diet. Just a thought. Make sure to talk to your doctor first.