Research has shown that what we say only accounts for 7% of the overall impression we convey to others. The majority comes from our body language, followed by the way in which we speak.
With this in mind, its advisable to make sure that you're not giving out any negative signals. Examples include sitting cross-armed, talking with your body facing away from your date or sitting upright and rigid. You want to appear calm and relaxed. A good tip is to mirror your date's posture as this will subconsciously make them believe that you're both alike.
Also, be aware of their body language towards you. This will give you an indication as to whether you're hitting it off or not.