If your child will start preschool next year, start gathering supplies and appropriate clothing now. Take your child's growth rate into account, of course, but get things now so that your child can get used to wearing them. The right clothing will allow the child to play and sit comfortably while also making your life a lot easier by not adding more work to your childcare duties.
Whatever you send the child in to preschool has to be washable in a regular washer and dryer. You do not want to send anything that has to be hand washed, spot cleaned, dry cleaned, or any other cleaned to a preschool. Your child will be playing with crayons, pencils, glue, dirt, and whatever else he or she finds, and that will get onto the clothing (and that doesn't even begin to describe the food stains you might find). You want to have your child wear something that you can toss in the washer when he or she gets home.
You also want to find items that are easily replaced if they are lost or become too damaged to clean. Plain T-shirts, comfortable pants, plain socks, and so on are appropriate. Your child's favorite one-of-a-kind, knitted-by-Grandma sweater does not belong at preschool if you're worried about losing it.
Everything your child wears at the school should be labeled, especially items of outerwear and shoes. Anything your child can or has to remove at the school needs to have a label so that teachers know what belongs to whom.
Closed-Toed and Heeled
Shoes should be closed-toed and closed-heeled. Sandals may look cute, but they can catch on things and make children trip. Flip-flops are a bad idea -- they can catch and twist unexpectedly, leading to foot and ankle injuries. Open-heel shoes can fall off.
It should go without saying, but many buys parents don't realize that clothing has to be weather appropriate. Even if the weather is supposed to get better during the day, ensure your child has enough so that he or she is always prepared for whatever weather occurs.
Many preschools you look at will have additional dress code requirements. Granted, they aren't going to be much considering this is a preschool, but you should abide by them. Ask the school about any conflicting policies before the school year starts or before your child joins the program.