Change your ads regularly. Your prospects could get bored seeing the same ad all the time. Statistics show people usually see the same ad 7 times before they actually buy. Just change them enough to keep them from being over-exposed.
Lower the chances of negative word-of-mouth marketing. You'll always have customers who are dissatisfied. Try to please them as much as possible. You could give them a refund, discount, a free product, a coupon, a rebate, a compliment, etc. Just be polite and stay calm if they are angry or frustrated with your business.
Give people a deadline to order. Tell people if they order by Sept.15, 2002, they will get a discount or free bonuses. This will create an urgency so they don't put off buying. Another example, "Order before 8:00 p.m. US/EST and get a second product of your choice for free!"