"No money down." That's the pitch Donald Gilligan used to make as a salesman for an energy service company. Once upon a time, it was the overture to an energy-saving spiel that was very appealing. But another company, Enron Energy Services, came along with a better-sounding pitch: a cash advance on the energy savings that were sure to come. After Enron Corp. collapsed, promises of cost-free energy savings came to sound hollow to building owners who might otherwise have been interested in the services Gilligan offered. Now, the energy service industry is trying to rebuild its reputation.