Placing a collar of carpet underlay around the neck of a young cabbage will prevent the cabbage root fly from laying its eggs at the base of the cabbage. Placing a bottle cloche, a clear plastic drinks bottle with the top and bottom removed, over newly planted vegetables will prevent them being eaten by slugs or anything else that takes a fancy to them. Small gage chickenwire is always useful. Placing it over your newly sown peas can stop them being eaten by mice while they are germinating or being scratched up by cats. Wrap it around your flowering bulbs to prevent squirrels from digging them up.
Netting can be very useful at preventing bird damage to fruit and vegetables.
There is also a humming line that can be wound around canes criss-crossed over your vegetables to prevent bird attacks. Netting can also prevent cabbage white butterflies from laying there eggs on your brassicas. Barriers can also be used to prevents diseases. For example, peach leaf curl is a devastating fungus that can simply be prevented by placing a barrier of polythene sheeting over a trained peach tree in the winter. This simple barrier prevents the spores splashing up onto the plant.