Raised beds generally improve the drainage for your garden plantings simply by the fact that they are elevated above the surrounding grade. However, if the subsoil is dense and impermeable--such as hard-packed builder's fill, or heavy clay--and the raised beds are tightly built, they may turn into muddy pools under heavy rainfall or spring snowmelt, unless they have proper drainage. Install drainage channels in the underlying soil before you fill and plant your raised beds.
Dig a trench 10 inches wide and sloping from 8 inches deep to 12 inches deep from one end of the raised bed location to the other, using a trenching shovel. Dig a round hole, 12 inches deep by 2 feet in diameter, at the low end of the trench.
Place 4 inches of coarse gravel in the bottom of the trench, maintaining the slope.
Cut nylon window screen with scissors into roughly 10-inch circles. Secure the circles of window screening over the ends of the PVC pipe using large rubber bands. Lay a single length of 4-inch-diameter perforated PVC drainpipe in the trench.
Cover the pipe with gravel to grade level, and fill the hole at the end of the trench with gravel to grade level.
Cut 12-inch-wide strips of nylon window screening with scissors. Lay these strips over the gravel-filled trench, tucking the side edges down along the outer edge of the gravel to help keep it in place.
Fill raised bed with desired planting mix, wholly covering the drainage trench, and plant the bed as desired.