It is hard today to be a seller. They are competing with foreclosures and short sales, most aren't getting many showings and when they do, they get excited. They need a little TLC. What they don't need are agents who set up appoinments and then don't show their homes.
When you bring a buyer to a home and they decide from the outside that its not going to be of interest to them, don't just pull back out and leave. Take 2 minutes, get out of the car and go into the home. Leave a card to let the seller know that someone did show up, don't leave them wondering if they got home too early, if someone might still be coming by, or if maybe they got the day or time wrong.
If you aren't comfortable asking the buyer to come in for a moment with you, at least tell them that you will just pop in out of professional courtesy to make sure no candles or lights are left burning and to leave your card so the seller will know you came by. Don't be thoughtless, these homes aren't all empty foreclosures where no one cares if you came by or not, treat the ones that aren't with some old fashioned respect. I'm sure your buyers will appreciate your efforts to be a responsible agent and not resent the little bit of time it takes to do the right thing.