Keep refuse and garbage bags in bins, sheds, or the garage until they're ready to be picked up by waste management.
Use a rock to weigh down trash cans, or tie down the lids with rope. Otherwise, scavenging animals like raccoons or foxes may tear into garbage bags, spread trash around and attract seagulls to feed on it.
Make your roof uninhabitable for seagulls. A number of options are available, including spiked strips and plastic owls. Wire netting in and around chimney pots has proved to be moderately successful. Without a place to perch and nest, gulls may look elsewhere for a meal.
Don't feed seagulls. Human activities are responsible for giving gulls a number of feeding options, but intentional feeding causes the most problems. Feeding them attracts more gulls, puts them in competition with each other and leaves your home as a future target the next time they're looking for a meal. The birds are adequately equipped to locate their own food sources and do not need your assistance in doing so.
Contact your local government council, and ask if they have gull-proof bags available. These are specially designed to ward off the pecking and tearing that gulls do, and are a practical option for areas that do not use or provide wheeled bins with lids.
Spray garbage bags with vinegar or an inexpensive furniture polish as a deterrent measure, in case your garbage bags somehow get torn open.
Clean out disposable food containers thoroughly before placing them in the garbage.
Throw a sheet over your garbage bags, so they're not in immediate view.