Note: I orginally wrote this post for the Mighty Girl website - but as Andrew so rightly points out in the comments - this is great advice for any time we have to be away from those we love, no matter our ages.
1) Acknowledge the child's feelings.
Empathize with them. "I know, it's hard to be apart, isn't it?" And normalize their feelings. "Everybody feels a little bit scared when they go to a new place." Let their attachment to you be a place they can rest in love in the midst of their anxiety.
Do not minimize the child's feelings or ask them to change how they feel. "Can you be a brave big boy for mummy?" Do not exploit their attachment to you by trying to manipulate their feelings and behaviour. Don't make them work for your love.
2) If the child is going to school or childcare, let them see you interact with the teacher or the caregiver in a positive way.
Children are naturally wired to be wary of strangers - for good reason. They will, however, take cues from those they love as to who is worthy of their trust. If they see you interacting with the teacher with smiles, nods, laughter and even a hug, if possible, they will be able to feel safer with that person. Not that they will bond immediately, the relationship will still need to be developed, but this provides a good footing.
3) Give the child an object through which they can feel connected to you while you are apart.
A scarf that smells like your favourite perfume. A locket with a picture of you and them inside. Matching bracelets that you both wear - these can be a simple as a coloured string - hey it works for Kabbalah peeps! Imbue the object with some magic powers, "When you open the locket invisible magic dust will come out and you will be able to see Mummy in your head and mummy will be able to see you in her head, and it will be just like we were together." "There is an invisible string connecting our two bracelets and when you tug on your bracelet it will travel along the invisible string until it gets to me."
4) Focus on the return
Don't talk details about the separation, but give details about the reunion.
"Oh, it's going to be so wonderful when I come to pick you up. I'm going to give you the biggest hug and smother you in kisses and we can cuddle on the couch. I'm going to be so happy to see you!"
5) Don't avoid the goodbye
It is very common for parents to try to sneak out of the house or away from the school and avoid dealing with the feelings of separation altogether. While understandable, it is much better to focus on developing emotionally healthy separation rituals then to leave the child feeling abandoned.