Know your child
Perhaps the most important thing to do before researching preschools is to jot down a few notes about your child. Ask yourself what he likes/dislikes when it comes to interacting with others, learning and playing. Think about what you want him to get out of his preschool experience.
Do your homework
Find out about a preschool's mission before entertaining it as an option for your child. Is it academically focused? Based on play and social development? Also consider the classroom timeline and how your child might best succeed and enjoy his time -- in a structured class environment or a less rigid one. Many in-home preschools were born out of a mother's desire to continue nurturing other children. This may be an ideal place for your child to segue into his first school experience while still remaining in a loving home environment.
Take a tour
Always take a tour, meet the directors and the teachers and get a feel for the preschool before laying your child's tuition down. A mother's instinct is a powerful thing -- trust your gut.
Consider parental participation
Some preschools offer a co-op format where parent participation is a big part of the preschool experience and maybe even required. Others ask for parent participation in the way of occasional (and optional) volunteer opportunities in the classroom or helping out with snacks and supplies. Find out what is expected of you before you sign your child up for a particular preschool.
Figure in all the factors
Aside from everything else you'll want to consider when searching for the perfect preschool for your child, consider realistic factors such as cost and location. If you pick a preschool that's miles away from your own neighborhood, it's less likely that your child will continue on in school with his first friends when he goes off to Kindergarten. Plus, make sure you'll be able to get your child there on time every day and pick him up in a timely manner as well. If cost is an issue, but you have your heart set on a certain school, speak with the director about spreading out payments or ask about scholarships.