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Professional painters spend far more time on preparation than on actual painting. Fill every hole and crack with spackling, let dry, and sand smooth. Clean walls with a damp cloth to remove dust and grease — paint won't adhere properly to a dirty surface. Apply painter's tape along trim, baseboards, and ceiling edges carefully; press the tape edge down firmly with a putty knife to prevent bleed-through.
Primer is not optional when painting over a dark color, bare drywall, or fresh patches. A coat of PVA primer seals patches so they don't show through as dull spots in the final color, and tinted primer (ask the paint store to tint it toward your topcoat color) cuts the number of finish coats needed from three to two.
Paint in the correct order: ceiling first, then walls, then trim. For walls, cut in the edges with a 2.5-inch angled brush before rolling — cut in about 3 to 4 inches from all edges to give the roller room to work without touching trim. Load the brush generously but not dripping; feather out the edge with light strokes.
Roll walls in a W or M pattern, starting from an upper corner and working in 3-foot sections. Keep a wet edge by overlapping each new section into the still-wet previous section. Two coats of quality paint almost always beat three coats of cheap paint in both coverage and durability. Allow the first coat to dry fully (at least 2 hours for latex) before rolling the second.