Make sure you use AE, a meter, or know light like the back of your hand. [/QUOTE]
Meter incident, if you can. I don't recommend AE unless you tightly control it.
Without an incident meter I find it's better to know light like the palm of your hand. That is, the method I use to ensure good exposure with slide film is to meter off the palm of my hand and open up a stop. Or read directly off a gray card. Both are effectively similar to incident reading the light, which I consider for slides superior to reading reflected light.