I went through just this last weekend, and my situation is similar to what you describe. I'm far from experienced (we have one ride of thirteen miles under our wheels), so I'll humbly offer the following in the hope that another newbie's experiences prove useful to you:

- The benefits of clipless pedals are obvious the first time you use them. Similarly, platform pedals offer a number of drawbacks (feet falling off the pedals; permitting the stoker to use legs as outriggers when balance-panic sets in; difficulty getting the captain's pedals in place so he can clip in; etc.). My wife had never used clipless before and was a bit anxious about them. So, what we did was spend some time in the garage, me straddling the bike, and she practicing getting clipped in, seated, and unclipping. It took her a fair amount of trial and error for her to line up the cleat with the pedal, but after a while it became far easier and, honestly, halfway through our first ride, became nearly second nature. (Also: should the unfortunate happen, I think it's much easier to get out of clipless pedals than it is straps.)

- Read the resources cited in other threads; read them together and discuss. You really want your stoker to be comfortable before setting out -- possessing some basics tips about tandeming and stoking will make it much easier for her to relax and enjoy the experience.

- In a neutral setting (say, a garage or driveway), spend some time discussing and practicing the basics -- mounting (captain straddles, stoker mounts/clips in, etc.), figuring out where your preferred clipping in pedal position is, how you want to shove off (counting down worked for us). I'd go further and suggest doing this entirely separately from any ride you decide to take, to work out kinks and remove pressure (performance anxiety) from your first time riding.

- For the first ride -- literally, your first time riding the bike -- find a wide open space (a high school parking lot worked for us) for practicing. Not just getting under way, but shifting, turning (she will likely not take naturally to leaning while turning, and you don't want to discover that taking a sweeper at 40mph!), and stopping (no more track-stands for you!). We devoted about 10 minutes to the parking lot before hitting the open road, and it was time well spent.

- Talk/ask questions while you ride -- How's this pace? Is your seat comfy? Want to coast a bit? Don't just "go!".

- Have fun! (Else, what's the point?)

David
"Two-up since 4/12/09!"