Remember - every child is different. So there's never going to be a 'one size fits all' approach. You're always going to need to take the age and temperament of your child and your family circumstances into account.
I believe in the value of a good routine. It helps children feel secure and they know what to expect. If you're planning on sending your child to preschool it's likely this is going to mean a change in your child's day-to-day activities and it helps to prepare for that as a family. Your child may need a lot of reassurance that you will be there for them as before. Stay positive in your approach. Visit your nursery or pre-school beforehand to make sure you're up-to-date with logistics, regulations, health and safety what is expected of you generally and what your child will need to have in their nursery bag. Preparation is the key to success. You can't predict how your child will react to everything but there are many things you can organise to make nursery or pre-school days less stressful.
I'm writing to the title: 'How to prepare your child for preschool' but it is often the adults in the family that need to do most of the preparation. Getting ready for preschool means that we 'grown ups' not only need to get to grips with the practical issues but also need to engage in a little soul-searching - which includes asking ourselves the following questions: Are we emotionally ready to leave our children in someone else's care at pre-school? And if not, why not? Is it the right time? Would it be better to wait a few months until our child is older? How many hours do we feel comfortable with? Of course - sometime the parameters are defined by our paid work - but once we have answered this questions we're likely to feel a lot more comfortable and confident about our choices and that's going to rub off on our children.
In a good nursery or pre-school you will have the chance to talk to the staff about their approach to early years 'education' and what happens on your first day. It usually helps to talk to your child in advance about the interesting things they can do at nursery (that they might not be able to do at home) - new friends they make and things to look forward to. Nutrition and sleep are particularly important, so think through 'lunchbox' issues and stick to reasonable bedtimes in your daily routines.
When you drop your child off at preschool - settle them in, give them a big hug - tell them you love them and give them some idea of when you will pick them up (after lunch, in the afternoon - something they will understand easily). Unless they are visibly distressed or struggling - make a move and be confident about getting on your way. You can usually 'wait in the wings' for a while out of sight and check in with pre-school staff to reassure yourself you child is doing fine.