I set up our snack as a monitored center. The the main reason I monitor it is for state record keeping reasons. Children choose whether to eat snack or not. I do this because I feel children are capable, and it builds self confidence. I trust them to know if they are hungry or not.
The first time I tried snack as a center it failed, mainly because I set it out too late and everyone was hungry at once. I ended up cleaning off all the tables anyway. After rethinking, I set up about a half an hour after arrival and left it out until everyone has eaten or has told me they don't want snack. The first couple of days it stayed open for about an hour. Now it stays open for about a half hour as children figure out what works for them.
This way I have less transitions, more choices, and fewer self-created discipline issues.
To set up snack as a center, I set out cups, napkins, and bowls. After they wash their hands, children are instructed to take one of each and bring them to the table. They then use a small milk pitcher (it handles about 10oz) to pour themselves a drink. If they are new to pouring, I instruct them to use two hands, one on the handle and one on the spout. I tell them to pour slowly and stop when they think have enough (not necessarily when they get near the top). They can always pour more if they want more. An adult is nearby to refill the pitcher as needed.
We had one spill the first day because the child was very excited and not concentrating (a skill unto itself!). I asked the child to get some more napkins to clean up her mess and she did by herself and then tried again. The second time was a success! :)