Water is a great wildlife resource, so a pond or even a small water feature provides a valuable addition to the garden.  Make sure you include a shallow area to allow escape for hedgehogs and other small animals if necessary!
Avoid the use of slug pellets, which are another hazard for hedgehogs and can also poison birds. Go instead for non toxic pellets, ‘slug pubs’ or biological control with the Nemaslug.
Make a hole for hedgehogs! If your garden is surrounded by close panel fencing or walls, making a small passageway to a neighbouring garden will allow nightly transit for the animal
Good wildlife gardening means aiming for a balanced ecosystem - rather than going for toxic chemicals, rely on natural predators to help deal with potential pests.
A wildlife garden should also be a sustainable garden, where scarce resources are conserved and materials reused or recycled wherever possible.

See also our water-saving tips for gardeners.

 Some useful web-sites:

Grow a variety of flowers to attract bees and butterflies - early and late flowering plants provide nectar for insects at critical times. Single petal varieties, especially annuals, will be more useful to insects seeking pollen & nectar than multi petal hybrids. Leave tidying of borders and shrubs until late winter/early spring to maintain cover and retain seeds and fruit for birds and small mammals throughout winter. Create homes for a variety of invertebrates by making a log pile, or build a ‘bug hotel’ using old pallets or cast off untreated timber, bamboo canes, old bricks etc. Solitary bees will find a home in a bundle of canes hung in a sheltered spot. More simply, a plant pot filled with dead leaves and turned upside down, or flat stones placed around the garden will give mini beasts places to hide!

Compost heaps/bins are not only good for enriching your soil but provide nourishment for many small creatures as they recycle your waste. If buying compost, avoid peat based products.

Trees - especially native species and fruit trees - are a great attraction for wildlife and can be accommodated even in small gardens with careful choice of variety. 
If you have room to leave dead wood in place it will nurture beetles, fungi & mosses.

Any garden, big or small, can be home to a variety of wildlife. In general, the key is to provide as many habitats as possible, with plenty of shelter, foraging opportunities, and if you can, some water. Lawns are good for Blackbirds and Starlings that search for leatherjackets in short grass; if you avoid use of weedkiller & artificial fertiliser you will allow clover and other flowering species to provide foraging for bees, and seeds for Finches.

Downton green network

A patch of long grass will offer habitat for beetles and overwintering caterpillars. If you can host a nettle patch, you are providing a larval food plant for Small Tortoiseshell and Peacock butterflies. Create shelter by growing climbers against walls (ivy is great) so that birds may find roosting and breeding sites; berry bearing shrubs will also provide food. If you have room, a native mixed hedge will sustain many different species.