I've used several different approaches to this.
In order of preference -
1) Manfrotto Superclamp with Manfrotto 196 articulating arm on the arm rest - if for some reason the arm rest is shaped so that you can't make it work then
1b) Superclamp + 6 extension + Manfrotto 099b extension pole clamped below somewhere on the seat base support.
2) Windtech C clamp with 24 inch flexible gooseneck (the firm kind not the real floppy kind)
3) unopened stand gaff taped to the seat - I almost got tossed from a nice theater once when doing this. A head usher got freaked out thinking I was using duct tape in their historic theater, gave me a real hard time and threatened me with expulsion and damages. I asked him to radio anyone on the production crew and explain I was using gaffer's tape. So he did, right there standing over me, and said "Oh, so gaff tape is fine? Are you sure?" Then told me to have a good night and walked away.