Learn as much as you can about the material you are performing. Tell something about the history of a song, the guy or gal who wrote it - what your grandma did the first time she heard it, whatever.
A footnote to the "story" hint above - people love stories. A lot of songs are stories, but even story songs have stories about them. I have heard John Denver tell some stories behind his songs - and the songs are a lot more meaningful now. I heard Marty Robbins wrote "El Paso" as his wife drove along through the town and out into the wasteland of southern New Mexico. Does that hit home with the local folks? You bet. Point is, the audience then connects with the song, and by connecting with the song they connect with you. Don't overdo it (tell a complete opera about every selection) but a well timed historical or hysterical vignette will have the audience eating from your paw (hand) and begging for more.
If you haven't heard some of Seamus Kennedy working a crowd, buy some of his cds. One show in particular Seamus was on stage (outdoor setting, picnic atmosphere) when this guy walks past the stage in the loudest "Hawaiian" kind of print shirt that you can imagine. Seamus stops what he was saying in mid sentence, lets the guy move front and center of the stage and then says, "I have a set of pajamas that look just like that." The audience roared for 5 minutes.