Make sure you technique is good and there is no tension in your handling. Your timing should be that you get to the force card right as they are about to select it. This is important, because if they see the deck spread out in front of them before you ask them to pick the card they have time to think "How can I be a pain in the neck? Maybe I can pick a card from the edge." The sequence goes like this: 1) you position yourself about 3 feet from the spectator and say "Take a card, it doesn't matter which one"; 2) you start spreading the cards with you arms at your side, elbows at a 90 degree angle; 3) their hand goes up to take a card; 4) you arrive at the force card; 5) you push the spread towards them by extending your arms -- essentially putting the force card a the tip of their finger; 6) look at them and not the cards as they take the force card. Try to keep talking as the selection process is going on (it makes it look less important, keeps attention focused on you and provides little attention on the spectator which makes them less likely to show off by picking the card second to the top.
Your attitude should be that it doesn't matter which card they select (and it doesn't because if they don't take the force card you have another effect you can perform). "The effect hasn't begun yet. This is preliminary. It is inconsequential. There is 52 cards and any one of them will work fine." That is your internal silent script. If you don't care which card they take, the audience (and spectator) won't care either.
Perception of reality is a selection of reality which results in a distortion of reality.