Master formula: $f' = f(v \pm v_o)/(v \mp v_s)$. Numerator: observer terms. Denominator: source terms. Signs: approaching = frequency increases. For numerator ($v_o$): add if observer moves toward source. For denominator ($v_s$): subtract if source moves toward observer. Mnemonic: "Observer on top, Source on bottom. Moving toward = bigger frequency = + in numerator, - in denominator."