http://www.colorado.edu/conflict/peace/treatment/activel.htm is a good start.
We could add that our reflection of what they say should arise out of a sense of genuine curiosity: "What I think I'm hearing you say is [X, Y, Z, etc.] Am I getting that right?" [or "Am I anywhere near right?"] We're very interested in any clarifications or expansions the person may have. There's a humility involved, a realization that we may well not have it right, and a genuine desire to know.
Sometimes (as in Dynamic Facilitation) this is combined with chart pad recording. In this case, it becomes easy to fold active listening into our note-taker role: "Check what I've written here [then read it out loud]. Does that capture the essence of what you're saying?"
Also we can go beyond just saying "I hear you're frustrated." We can include the sense of frustration in how we reflect what they said; we can actually take on the frustrated role, using the language, emotion, tone of voice and body language that a frustrated person would use. This helps them feel we're really "with them," getting not only what they said but "where they're at."
See also the notes on feeling heard at Underlying Dynamics Of Process.