Smile, show interest and lean towards them (slightly) when they speak, face your body and feet towards them as you talk, put some thought into your outfit and groom yourself up well, and compliment them. Small things like twirling or messing with your own hair is considered flirtatious also on a small scale.
Well ain't you smooth Sustained eye contact has to precede hair twirling. If you're avoiding eye contact and twirling your hair, it'll look like some sort of fake appeal to an easy let down. But if you look away out of shyness, that's usually visible too.