Maybe you've moved a picture, or accidentally spilt something that made its way to your walls. Scrubbing dirt off your walls may also leave areas worn down to the base coat. For each of these cases, you'll want to do a bit of touch-up work to blend in the repair.
When you complete any paint job, store away the leftover paint use for touching up. Seal the can tight so it won't dry up. Even paint that's thick with a bit of age can be brought back to life with water, extenders or solvents, for either oil or water-based paint.
Try cleaning the area with a little soap and water and a plastic pad that won't harm the finish. Avoid heavy-duty cleansers which will destroy the colours and surface of your paint. Let the area dry overnight before making your decision to move on to the next step.
Use a feathering technique to blend the new paint with the old. Dab some fresh paint into the centre of the repair area. If the area to be touched up is less than 3-feet square, spread the paint from the centre outwards. Use a brush if that's what you used when you first painted the wall. Otherwise, use a mini-roller. For a larger area, apply paint to the wall up to the edges of the repair spot with a paint brush. Run a mini-roller over it to create texture, running the new paint over the old.
If the paint job was done several years ago, it will probably be necessary to repaint the wall. The only exception might be if you've used an off-white colour and can replace it with the same colour from the same manufacturer. Colours fade and you'll get the best results if you repaint the whole wall, making sure you don't touch neighbouring walls with the new paint.