I don't ride pacelines often either. Once in awhile I ride with a group that does a loosely organized paceline. At first it was hard for me to remember to use hand signals and to shout 'car back', but I got used to it pretty quickly.
Much of paceline etiquette seems obvious to me -- soft-pedal instead of braking; maintain a constant at the front; don't allow gaps to form; ride smoothly and predictably. And if possible, stay on the front at least as long as everyone else does.
In general it's rude to hitch onto a paceline without asking permission -- even if it's just one rider. Besides, most rides I want to complete without any help. There have been rare occasions when a group of 2 or 3 just barely manage to catch up to me, and I'll mix in with them and chat for a bit; but usually the speed differential is enough that I just let 'em go.