Wearing thin? This nightly, leave-in treatment fattens and fortifies over time for thicker, stronger strands and fuller hair (really). Who: Those wearing thin, the fine, and the ultra-fine. When: Nightly. Twice-daily for the ultra-fine. How: Before bed, pump serum into palms 2 or 3 times; work into roots and through damp or dry hair. Longer hair types should pump again and be sure to smooth through ends.