Always explain to your child the reasons for them coming to this strange place: you're returning to work so during this time they're coming to the crèche to have fun with the other babies. Of course, your little one won't understand the literal sense of your words but they'll understand the context; they're responsive to reassuring messages you give them.
As you leave, say goodbye and assure them that you're coming back for them at the end of the day.
Leave them a scarf or an item of clothing that you've worn which has your smell on it and request that it's put in their bed with them.
On the first day, explain to them again why they're there, hug them, say goodbye and then leave straight away. A prolonged farewell will only worry them and will probably lead to tears...
Don't fret yourself - they'll be fine! Be aware of your own reactions: the chances are that the ordeal of separation will be more difficult for them than it is for you.