Many, many times...
1- Check with the backcountry desk the day before you go for any update info. Rangers and other hikers go thru almost every day and will usually leave info on changes at the backcountry desk (you need to go there anyway to get your permit).
2- Make sure you've got a good route description. The entry isn't hard to find but every year I hear about a friend who missed the entry and ended up walking around for hours trying to figure out how to get down into the canyon.
3- Take a short lenght of rope. There are a couple of "climb down" spots and a rope makes them much easier (and less dangerous). Often there are ropes in place but you never if they will be there and then you don't know how long they've been there.
4- Get a small dry bag (maybe 2). They're cheap and they are much more reliable than ziplock baggies.
5- Keep some dry shirts in your dry bag. The water is often very cold and small or skinny people can get cold if you stop for lunch (memories of my friend's son shivering).
6- Be prepared to swim. There are a few spots where swimming is not optional. IIRC there are very few of them and they are short but things change.
7- Keep in mind that the "hike" out is actually a very steep (but not very long) switchback trail. It can be difficult after a long day in the water. Some people have trouble in soggy Teva sandals. I usually carry a pair of cheap sneakers in my dry bag for the hike up and out.
Check with Tom's guide
http://canyoneeringusa.com/utah/zion/subway.htm