Another solution is to duplicate your background layer, or if you have multiple layers, create an adjustment layer (he button on the bottom of the layer palette that looks like a half-moon, Levels, Curves or Hue/Saturation will do fine, just don't use something like Color or Invert) and just hit 'OK' without adjusting anything. With the new layer on top, set its blend mode (on the layer palette, at the top, it reads 'normal' by default) to 'Multiply.' Now, it may be a bit too dark at this point, which is easily fixable by changing the new layer's opacity.