You would not believe how many customers call asking me to replace a few balusters that have been broken on their staircase. In most cases "horse playing" kids are the responsible culprits. (The only reason my kids did not break any balusters is because we didn't have a stairway.) anyway This Old House has produced a video describing how he does it (or would do it).  It's an excellent method for the experienced DIY person or professional.  You can view it here:  This Old House