Go for Her Neck!
According to William Cane, author of The Art of Kissing (which I have not read), 96% of women say they like being kissed on their neck.
Ninety-six percent is a very large number – the odds are high your wife would enjoy having you kiss her neck. Cane also points out only 10% of men like to be kissed on the neck, which is no doubt why so few men think to kiss their lady there.
Give it a try. Come up behind her and go straight for the side of her neck. Or, while kissing on the lips, work your way down to the area under her ear. On the firmer parts of the neck, you can also try nibbling – some women find this a huge turn on. During foreplay consider the neck a must visit stop between her lips and her breasts, and see if she warms up more quickly.