Evaluate the risk by considering how much you value the garment. Disobeying the “Dry Clean Only” label means you are washing it at your own risk.
Check the fabric to determine if it is likely to shrink when wet. Depending upon the fabric, it may be very durable to gentle washing in cold water. Some wools will shrink immediately, while pure cashmere may endure a cold water rinse with very little shrinkage. Polyesters and nylon often are unaffected by water, but if they are sewn with cotton thread, the thread may shrink when wet warping the garment.
If using the delicate cycle on a washing machine, you might want to consider using a nylon mesh laundry bag to prevent twisting during the wash cycle. Laundry bags allow water and detergent through while preventing your clothes from damage from intertwining. (Buy mesh laundry bags here.)
When handwashing, always use cold water. Dyes can bleed in water, so take care not to combine garments in the same load that could be stained.
Wash only one item at a time. Fill a sink or tub with cold water. Add a few drops of a mild detergent such as Ethique Eco-Friendly Laundry Bar or Dr. Bronner’s Biodegradable Cleaner. You do not want to use so much soap that the fabric suds, so spot clean but keep the water very lightly soapy. Gently swish the clothing back in forth for only a minute or two.
Lift out of the water, then rinse in clear cold water until all soap has been rinsed out. Do not wring dry, but rather gentle squeeze out the water. Lay flat on a towel to absorb more water. Then lay flat to dry out of direct sun.