Join 500,000+ artists on ConceptArt.Org.
Its' free and it takes less than 10 seconds!
Put the Adjustment Layer and the layer you want to adjust into a layer group. Then set the blending mode of the group to "Normal" instead of "Pass Through".
This will isolate the adjustment to that group.
Alternatively place the Adjustment layer just above the layer you want to affect, then place the cursor on the line that divides the two layers in the layers palette: now if you hold down the Alt key your cursor will change to two little circles, click once and the adjustment layer will only affect the layer you want.