It's mainly about having multiple layers which have a different layer type (normal, overlay, burn, dodge, saturation, soft light, hard light etc.)
In a good-looking sig, there will be numerous transparent layers that have very subtle things (like paint brush strokes, other graphics, just fuzzy brushed black OR white) set to a different mode that overlays the basic layers of the sig (the background & the render... and maybe sometimes the text.) There also may be layers which are completely coloured with the paint bucket and set to a certain layer type to give the sig a certain lighting and/or colour.