Be the Back Up!
Many people avoid going into a back up offer position because they don't think they really have a chance to get that house they want. Don't be afraid of a back up position! If a seller is accepting back up offers, then they may not be very confident in the first buyer's ability to close.
We just had a buyer in first position back out because of financing trouble. We had an eager buyer in back up position that was willing to take the chance to get what they wanted... and they won. If you can go into a transaction and be patient and want the house enough to wait for a chance, you may just win by going into the back up position on a purchase.