Obviously, abstaining from pesticides is the best thing you can do to foster a healthy population of helpful insects. The garden will take a while to find its balance, and there's bound to be an explosion or two of aphids to test your commitment. Aphids have a shorter, faster life cycle than their predators, who take longer to build up their numbers.
Diversity is the key to attracting beneficial creatures. The more habitats you have - no matter how small - the more wildlife you will encourage. Try to include as many of the following as possible, without making the garden over-busy: pond, woodland area (just one or two trees is sufficient), "meadow" or a patch of rough grass, log pile, compost heap and a stand of nettles (to provide a crop of early aphids for the first ladybirds to feed on).
Grow plants with a range of flowering periods so that nectar and pollen are available for months on end. Plant flowering herbs and annuals among vegetables to entice predatory insects, who will help control pests.