If eyes are the window to the soul, eyebrows are the curtains," says Eliza Petrescu of the Avon Center in Trump Tower (725 Fifth Avenue, at 57th Street; 755-2866). "I can change someone's expression from angry to open." Petrescu has become so well known for her pluck ($45 to $65) that she is now marketing her own twelve-piece eyebrow kit, containing tools ranging from tweezers and eyebrow mascara to undereye cream and eyebrow gel, for $85. With the choice of who prunes your brows becoming as crucial as that of who cuts your hair, we point you to Pierre Michel (131 East 57th Street; 753-3995), where the considerably lower-key Dinah Aguirre's following is as thick as Frida Kahlo's unibrow. An initial visit will run you $50; a follow-up "maintenance" visit will run you $35. Independent make-up artist Paulo Siqueira (4 Park Avenue, near 33rd Street; 779-9270) believes waxing is too harsh for the eye area. Liv Tyler, Naomi Campbell, and Pia Getty trust him implicitly ($50 for an initial visit; $30 for follow-up visits.)