* digging and maintaining a pond in your garden – even a small, basic one will attract all sorts of wildlife
*make a small log/twig pile in a garden corner – lichens and fungi may grow, bugs and frogs will make a home and hibernate there
*leave a small, confined part of your garden to grow wild – plants like nettles and brambles are a haven for wildlife
*look into putting up bird, bat and bug boxes...
Excellent and detailed list of tips on gardening for wildlife here from Natural England.