A good tip is to save before you advertise in a certain area, and if you are really serious about it, save before each person until it lands you some good advertising... I haven't tried it but it might work..

And if you go to Chapter 5 and havent made it, just get 777 on the Lady Luck reels (very easy after some practice) and youll make plenty of Gil to buy credits and play games to bring up your advertising.