A few random thoughts on riding with a passenger:

There's no substitute for a bike with sufficient power, having taken
passengers very occasionally on the small (125 etc) bikes I used to
ride in the UK it was hard ! I'm now riding a GS1100 and that's great,
even moving from my freinds GSX600 I was riding recently. This may be due
in part to the fact that neither I, nor my regular passengers, are
particularly slimline !

I always tell passengers, stay in line with the bike on corners, that makes
their movements predictable.

My regular passengers both tend to hold either the rack or nothing at all,
I don't really like having a passenger hold onto me except for short periods.
Again that may be biased by the fact that I've had a couple of new passengers
(a friend recently for her first time on a bike) who hold on to me real tight,
making any kind of maneuvering difficult.

I've also discovered that on longer trips I prefer a passenger who moves and
shifts their weight a bit. I recently took a work collegue to a meeting (about
a 1 hour ride) and he sat totally still the whole way which left me also
sitting very still and getting a sore back.

We have a code system for turns, stop and for 'I need to shift position'.