You can reduce your returns by providing a size chart and detailed sizing information on the product. Include all the information about the product that you have in the description and post product pictures that clearly show the product.
Include a return form with packages asking for reasons for return. Does the product often run too large or too small? Make it clear in the product description that you recommend a smaller or a larger size.
There are products for which a higher rate of returns is to be expected, e.g. jeans. You can reduce the return rate with a varied assortment.