Smash the cake is one of the funniest experiences you can have as a photographer.
You'll need a pretty cake (professional look better), some subtle decoration and a gorgeous one year old eager to explore. Photographer with sense of humor is a must.
To be honest I didn't sleep well for a few days prior to my first Smash The Cake session. When you expect a carnage you are not quite sure how the situation is going to collaborate with your professional  perfectionism to photograph same pose over and over till you get the perfect shot. In cake cases you'll need to be pretty quick.
From personal experience I've learned:

• Prepare a bath for the end
• Dress baby in diaper cover or plain dress. You want the attention on the baby face expressions, hands and feet right
• Avoid chocolate cakes. Can provoke wrong associations when messed up
• Take some detail shots before putting the baby in
• Don't overdo with backgrounds or decorations
• Tell mum to give the baby cake at least once a few days prior to session. This is an important part and will give mum a hint if her baby has any allergies or reactions. Some babies don't like the texture of icing or cream believe it or not and may get very upset as it sticks to little hands. Others are seriously impressed and explore the cake thoroughly.
• Have a spoon handy in case the baby needs some help to smash the cake and gets it messy
• Keep some water for the baby at hand
• Let the baby be fed at least 30 minutes before the session. He wouldn't be hungry or grumpy when you meet him with the cake
• Have absolutely everything ready before you put the baby in
• Move around to get different angles
• It's usually very short. Unless you have 2 cakes
• Have fun