Clean your brushes well, and you won't have to buy new ones for every job
Expensive paintbrushes actually cost less in the long run than bargain-bin brushes because high-quality brushes will last a lifetime. But only if you clean them immediately after each use. Soak the brushes in either water (for latex paint) or mineral spirits (for an oil-based paint). Then rake out the bristles with a brush comb. Pay particular attention to the area where the bristles join the metal ferrule. Paint has a tendency to collect there and then dry in hard clumps.
Make cleanup easier on your next painting project by wrapping a strip of 2-inch-wide masking tape around the ferrule. Position the tape, as illustrated, so it overlaps the bristles by about ¾ inch; that will help keep paint from seeping into the joint between the bristles and ferrule.