Although there aren't many things you can "practice" per say about school before it starts (they're on their own when it comes to learning how to go through the lunch line and finding the bathrooms), helping small children physically separate from their parents can be a huge and essential step.
It seems like such a small ritual (unless there's a movie with an airport or train station involved), but learning to detach with a set number of steps or with a pattern is great for them and great for you too! It helps keep things short and sweet, giving them courage to go with their departure into the big brave world instead of thoughts of being away from home.
Try practicing at home, maybe you're headed to the grocery store or have an errand to run by yourself, say good-bye, throw them a hug and a kiss the same way you would before they go to school or hop on the bus and get that ritual seated in their minds. Likewise, you can also start similar rituals for seeing each other again. Specific questions asked or actions done that become a pattern to help prep their mind that you're not gone forever, just a few short hours while they learn their ABC's.
Have you tried this in your own home? Let us know your tricks for saying good-bye in the comments below!