OP here. Thanks for the replies.
Nice to know I'm not alone in the discomfort stakes on this one.
Have tried non verbal stuff (presenting my cheek *really* obviously). Expected she'd pick up on those cues but not so far (they've been married a few years). Have managed to dodge her a few times but she keeps doing it & has freakily fast reflexes or something.
Suddenly switching to a handshake (while a little tempting) would feel awkward at this point & probably hurt her feelings as well which I am trying to avoid. It is however something I've done with acquaintances who want to be more huggy/kissy than I prefer.
Think I'm going to have to say something to her and would really like some suggestions on what to say, how to approach.
Admit I chuckled over weeblewobble's suggestion that "If she protests that she wants some of that sugar, tell her that you're more comfortable with kisses on the cheek." Humour may in fact be the way to go.
ETA. CakeEater posted at same time. Hadn't tried the elbow grab. May have to rehearse as a manouvre or there could be some accidental knocking of heads.