(writing from a Photoshop perspective)
Add low opacity (say 10-30%) white lines under/right (i.e. offset) to all the black lines; it instantly adds depth. I'd actually suggest a series of these; for example have a 'top' set of darker thickness 2 lines hich create large panels, followed by 1-thick lines of a lower opacity creating smaller planels (equivalent in complexity to what you have now), and finally very-low opacity lines (only ~5%) creating very fine detail.
Also, I'd add in some layers of white, black, black grain (a paintbrush that looks pixellated/diffused and acts akin to noise; use it to cover everything in a non-uniform way) and possibly brown (very low opacity, as a sort of gentle rust/contrast effect) shading, using the paintbrush and with the existing black panel lines to define the shading (i.e. top & left is white shaded, bottom and right is black shaded, grain over both, and brown in the middle).
And possibly another couple of brief shading layers; more brown 'under' the thick black lines as a more visible rust effect, create charcoal under said lines as a darkening effect, burn tool for some hull scratches/dent effects, and a white paintbrush used on - IIRC - overlay mode to create the effect of visual highlghting (this is possibly less useful for tiles, but very handy for templated models).
This -
http://sectorgame.com/aldo/media/gtf_claymore.jpg - shows the sort of line arrangement and also highlighting effects, although 90% of the detail above should be visually indistinct; you want a sort of general effect rather than being able to point out x & y on the maps. I always took the approach that,in theory, no 2 adjacent pixels should be identical
