Keep a clean, wet cloth nearby whenever you paint, and wipe up paint splatters as they happen, if at all possible. Watch wood floors and furniture for the fine paint spray that can appear if you're using a paint roller on a ceiling. Wipe it right away, and dry the area with a paper towel to remove any excess paint film.
Clean up oil-based paint drips with a cloth dipped in turpentine, which will dissolve the oils and make cleanup easier.
Allow splatters on windows and other glass surfaces to dry before you try to remove them. Set the edge of a razor blade at a steep angle against the surface, and gently scrape the paint off. Wipe the razor edge on a damp cloth between uses. This method also works for wood and tile surfaces.
Remove paint from fabrics and carpet as soon as they occur. Dab upholstery or carpet with a cloth dipped in warm water. Run clothing or drapes under warm water and scrub until paint is washed away. After paint dries on fabric, it can't be removed.
Get rid of dried paint on metal surfaces with a heat gun. Heat will make the paint blister and peel. Wipe it away with a damp cloth after the metal cools off.
Paint over mistakes like over-painted trim and splatters on walls after they're dry. Prime spots if the color is sharply contrasted. For instance, a black spot on a white wall should be primed first before painting, to avoid having to apply a second coat for coverage. Allow primer to dry completely before painting over it.