For marred surfaces, try cleaning first. Good-quality paint can withstand some scrubbing. First try a sponge and dishwashing liquid, and if that fails, try a little household cleanser. In the worst case, the finish will be a bit duller, but you can burnish it with a soft cloth or touch it up with paint.
If you have made a spackling repair or are trying to cover a stain that won't scrub off, apply stain-killing wall primer over stains using a paint roller. Let dry. Apply the normal amount of paint over the repair area. Then feather the edges into the surrounding wall with a nearly dry roller by rolling out from the center with numerous light strokes. Lift the roller off the surface as you roll.
Apply a topcoat in the same manner, extending the paint to a loosely defined shape at least 6 square feet (.5 square m) in area. Clean your tools and judge how well the touched-up area blends with the surrounding area when it's dry.