An enhancement of some optical phenomena in photonicl iquid crystals at the frequencies of optical localized modes is theoretically studied. Along with the well known low threshold lasing and anomalously strong absorption at the frequencies of localized modes these phenomena also include studied here an enhancement of nonlinear second harmonic generation (SHG) in chiral liquid crystals at the edge of stop-band, SHG at the defect mode frequency, a nonlinear frequency conversion in a general case, sum frequency generation and Cherenkov radiation. It is shown that the analytic formulas obtained for the case of photonicl iquid crystals can be also applied for a qualitative description of the corresponding enhancement phenomena in conventional photonic crystals.