First tip is with your art tools, and practice with different media. For the competition I bought aseprite which had some nifty features I read about for making pixel art faster. I found it much better than using GIMP or Paint.net, especially the support for animating sprites.

Aseprite has an ink mode to the brush tool where you can click and drag a subset of the pallet, when in that mode only colours in the range are overwritten on the canvas, and by right and left-clicking you can shift the pixels one step up or down in your selected gradient. If you paint miniature models it's just like layering up highlights and shading on your base colour (but with no fear of painting outside your base colour).

I found by rapidly clicking left and right while dragging I could build up a wood grain texture quite fast, as used for the mineshaft lift.

Play Mine of Mine online here: https://ldjam.com/events/ludum-dare/48/mine-of-mine