Sagittarius is the sign of honesty, and so immediately the Sagittarian has to feel that you are being honest, and that's about yourself, about your opinions. Don't parrot other people's thoughts because your thoughts, your original thoughts are what the Sagittarian is interested in. Sagittarius is also the sign of adventure, and so if you have a spontaneous approach, say, yes, that sounds like fun, yes I'd love to do that, that sense of jumping in and experiencing life as an adventure and with spontaneity is very important to a Sagittarian.
Also Sagittarians tend to be intelligent and love to converse, so don't be afraid to speak and interact and not hide who you are. Sagittarius is the sign essentially that needs a sense of freedom, so immediately if the Sagittarian even if this is a first date, a first meeting, feels that you are the kind that is going to be clingy, or needy, or in some way hem them in, the Sagittarian will beat it to the nearest exit door. Sagittarius is a very good friend, and so basically that is where the relationship will probably end up. I don't mean not as lovers, but the component in a love relationship for Sagittarian is to have friendship and camaraderie of someone that he or she can play with. I use that word not as that you're playing with love, but that you're playing with life.