Tell an inexperienced pillion to sit 'like a sack of potatoes' and lean with the bike.

Be ready to be bashed on the back of the head under braking - a pillion needs to learn the art of not flopping forwards when you apply the brakes; it rather contradicts the 'sack of potatoes' instruction but as been said, the pillion holding the grab-rails helps and he/she should be encouraged to sit up with a straight back.
If you have a topbox with a backrest or just a backrest, it helps as the pillion can 'locate' against it and thus lean at the right angle when you're cornering.

I tell my pillions not to dismount until I give the instruction.

I concur with the other tips here.

I once told an inexperienced (and nervous) pillion to "lean with the bike as we corner". Unfortunately he interpreted that a bit too literally and, on the first corner, threw his full weight into it which came as a pretty unpleasant surprise for me. So now I always phrase it as "just let yourself lean with the bike" since it implies a more passive approach and has produced the desired result in every passenger since.

Just noticed this thread and I think everyone has pointed out the main issues. For me the biggest danger areas often occur at slow speed. The extra person makes the centre of gravity that much higher and simple things like stopping on a sideways sloping wet and greasy junction in town or manoeuvring the bike on full lock in a country lane with loose chippings is something you have to think about an plan far more than when solo. You can easily tip over. My wife actually fell off sideways during such a manoeuvre. –tbh I am glad she did because I could not have held the bike up for much longer with all that weight !
Next item to consider is a cheap intercom; our last wired one was a £8 unit from Lidl, so they ain't dear. The intercom lets you warn the pillion of approaching road conditions e.g. pothole. need for rapid braking etc. They can advise you of surrounding vehicles and their positioning on roundabouts and such like. An intercom prevents to trying to shout at each other while looking sideways and not forwards.
On major and country roads we find the bike far more stable than solo when pressing on, yes wallowing can be an issue with some road undulations and grounding is a danger because once the main stand grounds on a bump while cornering, the bike is pushed vertical and you are pointed towards oncoming cars.
Having said that providing the pillion just goes with the flow as my wife does, the trips out together on the bike are, well magic.

That's a very good point Meldrew. I won't ever let my other half on the bike unless she's wearing the proper jacket/boots/trousers/gloves no matter how much she tries to convince me! On the Xevo all the bike crap goes under the seat if needed so there is really no excuse.

So Mel,
How did it go?...

When I was a new rider and had inexperienced passengers on the bike it was quite challenging.
It's not the easiest thing. Be very careful when filtering.

XMax250Paul wrote:So Mel,
How did it go?...

We haven't taken the plunge yet. Been building up to it by buying appropriate, good quality gear. The missus won't let me spend too much cause she's afraid she won't like the experience.

So far we've got a good Ashman ladies leather jacket with armour (£22) Caberg Trip full face helmet (£58) and padded agv (£20, had to get those from a shop!) .