Break a toothpick in half. Get a small wad of 0000 steel wool and stuff it in the blade well.
Take the broken end of the toothpick and push the steel wool onto the backspring, and scrub back and forth. Do not use any oil when doing this.
Shake the knife to remove as much gunk and old steel wool fibers as you can, then tap the knife on a piece of wood to dislodge any further debris.
Blow out with compressed air, if you have it.
Lastly, a good flush with WD40, shake out as much as you can, then blot out the rest with cloth or paper towels. Dry, and apply a light oil to the blade joints.