This worked for me (us).
I asked my wife if she wanted to ride along with me for "just a bit" on my morning commute. (I invited her to come with me as far as our neighbourhood bakery—about 10 blocks—for a cup of tea and a hamantaschen) We kept to fairly level terrain, and quiet neighbourhood streets—or the seawall—and stayed clear of major traffic arteries, crossing them only at lights. At first she came along for just those ten blocks, and gradually became more comfortable with shifting and going a little quicker.
It meant a slower, more circuitous route for me, but we just left a little earlier. She really got to love being up and out early, seeing the sunrise, being in the quiet city. Soon we were shopping for a "nice" pair of cycling gloves (not lumpy, like mine) Now when she rides escort, when our paths part, she rides on for quite a distance on her own.
If you're just starting, be patient—through the reminders of which way shifts up and which shifts down—and encouraging. Mentioning how good she looks on a bike never hurts, either.