Try sharpening the artwork. Copy the whole drawing to a new layer, and filter>other>high pass (10 pixels). Set that layer to soft light, or other blending mode.

Old effect

Do you need an old/decrepit effect? Copy the lines, set the threshold pretty low, and color the layer red to add a messy, rusty effect to the lines.

Complexify the complexity

Do you need to use soft light/color dodge/multiply soft brushes to add pops of color or color tints? Play around, it can lead to lovely polished effect. Pump up light and shadow subtlety, add color complexity.