Do not wear cotton sarees. They will make you look more heavy. Wear silk sarees instead of chiffon to look in shape and thin. Stay away from printed sarees as they will make you look more heavy and short. I happen to read that handloom sarees look good on women who are a little heavy. Also you can drape it in a way that it gives you shape and make you look thin. Also, wear high heels that will give you height and you will look elegant.
You can go for dark toned georgette or chiffon sarees. Don't wear sarees with very broad and heavy borders. Also avoid wearing fluffy cotton sarees.