Be sure to use the same color paint and sheen to retouch the area. I always like to paint a little around the area too to blend everything
Also, using the same kind of roller as what used originally will help to result in a nice blending of the old with the new.
For small touch ups, I've had a lot of success using disposable foam brushes. Use a small amount of paint- almost a dry brush- and brush on the touch up in multiple directions. This helps feather the paint and blend new and old.
When I had a professional repainting of a wall after it was damaged she told me to use a small roller for touch up.
be sure that the sheen of the paint matches what you have on the wall, not just the color.