THE WOMAN BEHIND THE SEEN: Frances Olivia French, aesthetician and owner of Olivia's Institute of Beauty, 2025 Pennsylvania Ave. NW. TOP CLIENTS: Sharon Pratt Dixon, mayoral candidate ("Olivia's very skillful and effective . . . she's been very helpful in applying makeup for television"); Effi Barry, mayoral wife ("She's a line item in my budget"); Sugar Ray Leonard, boxer ("She does wonders"). TOP SERVICE: Facials ("my priority in life"), by appointment. OTHER SERVICES: Manicures, pedicures, makeovers, body treatments, waxing. WORK SHE WON'T DO: Men's bikini wax; women's sculp- tured nails. FEE: $45 and up. SPECIAL OFFERS: Five-hour "Beauty Day." Includes manicure, pedicure, facial, body treatment, followed by a light lunch. Starts at $135. PHILOSOPHY: "You can buy a dress to cover your body, shoes to cover your feet. You can get your hair done on a regular basis. But the face is one of the first things people notice. I'm here to help." FURTHER TESTIMONY FROM SUGAR RAY LEONARD: "It's one of the few times I actually relax and go to sleep . . . I become totally oblivious to my surroundings. It gives you a new attitude." REGRETS FROM SHARON PRATT DIXON: "I wish I could do it more often. I haven't gotten facials as regularly as I think I should. I have a rigor- ous schedule and enough stress, and it reflects itself on my face." A WORD OF WARNING FROM EFFI BARRY: "You have to be kind to yourself. We all lead very, very stressful lives. Women have to realize that it shows on our faces a lot faster than anywhere else."