Another option would be to use white acrylic paint. With my monochrome sketch cards, I use white acrylic paint for highlights, general clean-up, and some texturing. I get better results with the paint, as the white gel pen isn't opaque enough (I've gone through a bunch of these. The paint is still superior to it). If you try acrylic paint, I suggest investing in a small liner brush and practicing on sketches, if you're not used to using a brush. As Antlers said, use it sparingly, but when it's used, it usually makes for some nice results.