Avoid monocultures. This is where the same type of plant is grown en masse or in rows. Monoculture makes it much easier for pests and diseases to find their favourite plants and then spread quickly.
Plant herbs throughout the garden and vegetable plot, as most have strongly scented leaves which help repel insects.
Try intercropping. This is where fast-growing crops such as lettuce or radishes are sown between widely spaced rows of slower-growing crops such as Brussels Sprouts or parsnips . It utilises the space available and helps prevent weeds growing (weeds take nutrients, light and water, and spread disease).
Plant lots of insect-friendly or bird-friendly plants, either amongst your crops or nearby. They attract natural predators such as birds which eat slugs, hoverflies which eat aphids and bees which pollinate your crops. For a list of wildlife-friendly plants click here.
Take care with some companion plants such as mint– these are fast-growing plants and will quickly smother your crop. Grow mint in containers to keep it under control