The set of decays known as B⁰ → l⁺l⁻ are exceedingly rare. This thesis details procedures developed to optimize signal extraction and improve the upper limit on the measured branching fraction. When applied to 85 million BB events collected at the Belle experiment in Tsukuba, Japan, the resulting upper limits on branching fractions were 8.8×10⁻⁸ for B⁰ → e⁺e⁻, 1.0×10⁻⁷ for B⁰ → μ⁺μ⁻, and 8.5×10⁻⁸ for B⁰ → e±μ∓" at the 90% confidence level. / Ph. D.