Be sure to communicate to the employer that you thought it over carefully and appreciate their interest. This can be done in many ways. I've researched similar positions in similar companies and have found that to be the market range. You have the advantage of knowing the employer wants you. You must be able to express your reasons for believing you are worth the extra money without sounding unappreciative.
Salary negotiation is a business deal.
Maintain your professionalism at all times regardless of your dissatisfaction with an offer. There are other ways to bridge the gap between the employer's offer and your expectations. You might discuss the possibility of a three month performance review followed by a salary increase if your work is up to par. Perhaps you can arrange for an annual bonus or incentive plan which could increase your annual earnings. Consider your benefits package too.