Layers make everything more awesome. One of my favorite techniques for backgrounds is to scrape multiple layers of acrylic over each other, leaving it thick enough in some places to scrape through, kind of like sgraffito in ceramics. Sandpaper is also useful for distressing layers, though I didn’t use any here.
I have an empty ballpoint pen that I used to favor for details, but recently picked up a “colour shaper,” which is a rubber tipped brush that looks like a paper blender stick and a paintbrush made sweet music together (in a latex factory).
Choosing one from the selections of sizes, hardness, and profiles is kind of stressful, so be sure you bring a friend to help you out. (In art as in trips to the adult store, friends.)
Scrape on lots of color (old credit cards work beautifully, but so do folded up bits of cardstock or index cards), draw as you wish, let the layer dry, then go in again with a new one. This card has about seven background layers.
Tip: professional grade acrylics dry much faster than the student-y paints, so you have to work fast with those, but they also happen to be more intense and don’t warp your paper as much.
Watercolor of gesso makes for a brilliant top layer that scans and photographs particularly well, what with the light bouncing through and right back up and all.