One pressure point technique that I can vouch for working off the mat is what I call the "slimeball mustache (tm)". It can come in handy when you can't squeeze your arm under the chin enough to sink the rnc. It's fairly simple. Press your index finger over the persons upper lip, so that your middle knuckle is pressing into the divet (like your making a mustache over their upper lip with your finger). Then keep the pressure going into the divet, and also apply pressure upwards, against the nose. It's quite painful, and will usually make the person bend their head back enough so you can sink the choke.

--Chris
_________________________
"Seek not to follow in the footsteps of the men of old; seek what they sought."
--Basho