For many young children, preschool is their first exposure to a structured learning environment and the shift from home to school can be stressful for your child if he isn't ready for it. Start working on separation a year before preschool begins. As you do, make sure your child's time away from you is enjoyable for her. You might want to drop her off at a little friend's house or invite a favorite relative over for an hour or two of playtime. When she realizes she can have fun without you, she'll be more willing to let you leave.
Many preschools have large classes with rigid time frames and lots of group activities.
A child who doesn't have the self-control to sit still and be part of a cohesive group probably wouldn't do well in a preschool setting yet. Look for a sense of patience and an emerging awareness of others' feelings. An attention span of 10 minutes is good for a 3-year old. Most preschools require their students to be potty trained before enrollment. If your child isn't ready for school, don't worry.