At about six months, your baby will begin to realise how much they depend on you. It's a good idea to get him or her used to one or two other familiar people before this point, so that they can spend time in their care, apart from you. See 6-12 months: How your baby develops/How your baby feels.
When you need to return to work, or simply have some time off, it's obviously easier for your baby if they can be left with someone familiar, and be looked after in their own home. Even so, your baby may still scream and cry when you attempt to leave.
Don't be tempted to sneak out without saying goodbye in the hope of avoiding the screams. Your baby needs to register a protest and accepting the feelings (and surviving them) will make you both more confident about time apart in the long run. Keep the partings brief at first.
If you plan to leave your child at a crèche or nursery, get them used to the place beforehand by taking him or her along for a session or two with you. Make sure that they have a single 'named' worker to look after them.
Always leave your contact telephone number in case of an emergency. Leave a supply of nappies, wipes, change of clothes, and a familiar toy or blanket. A favourite cuddly or treasured book can do wonders for a smooth transition.