To define a sustainable yard, you first need to define sustainability. The Environmental Protection Agency defines sustainability as "meeting the needs of the present without compromising the ability of future generations to meet their own needs." WordIQ.com has a more detailed definition: "Sustainability... is intended to be a means of configuring civilization and human activity so that society and its members are able to meet their needs and express their greatest potential in the present, while preserving biodiversity and natural ecosystems, and planning and acting for the ability to maintain these ideals indefinitely."
What exactly does this mean for your yard? Your first priority is to have a beautiful yard now the needs of the present. However, in order to be sustainable, you'd have to maintain your yard in a manner that doesn't compromise the ability of future generations to meet their own needs and in a way that preserves biodiversity and natural ecosystems. There are a number of ways you can accomplish this, a few of which I will set out below.
One good way to preserve biodiversity and natural ecosystems is by landscaping with native plants. If you live in a relatively rural area, your yard will be contiguous with the surrounding habitat, rather than existing as a sterile island amidst a vibrant ecosystem. If you live in the suburbs or the city, your yard will exist as a habitat oasis or even as a wildlife corridor in the middle of miles of pavement. Naturescaping, or landscaping with native plants, is becoming a more and more important way of creating and preserving habitat as more land is taken up by sprawl and incorporated into cities.
An important way to keep your yard from compromising the well-being of future generations is to make sure your yard is one that takes up few resources, especially non-renewable ones. When you use less water on your lawn, more can stay in streams and rivers to preserve important fish habitat and more is available for drinking and food agriculture. Pick plants that are less thirsty and more drought-tolerant, and make sure to water when it's cool in the morning or the evening rather than the middle of the day. You can also invest in a drip irrigation system, which is more water efficient than sprinklers or watering by hand.
Other than water, you can also cut back on using gasoline and synthetic herbicides, pesticides, and fertilizers. Gas is easy to cut back on avoid gas-powered mowers, leaf blowers, and weed whackers. Use push-mowers, rakes, and hand weeding instead. It's a lot less noisy, a lot less smelly, and it's better for the environment. It's fairly easy to find organic and eco-friendly substitutes for synthetic fertilizers as well. Start your own compost pile and get free organic fertilizer in solid and liquid form in finished compost and compost tea.
The herbicide and pesticide habit can be harder to break. It's just so easy to spray and vola, your weeds and pests are dead. However, it's important to leave these toxic products behind. They can persist in the environment from a few weeks to a few months, and they can easily travel to other areas through runoff from your lawn and garden. Exposure to these contaminants isn't exactly good for your health either.
What can you do? First of all, learn to love weeding. It's been a part of gardening since time immemorial, and it won't be going anywhere any time soon. You can keep your weed crop down to a manageable level by hoeing down weed seedlings in the spring and mulching around your plants and beds in the spring and fall (good for keeping in water, too!). Also, make sure to get rid of weeds before they have a chance to go to seed, and do your best to remove the entire weed, roots and all.
There are also many sustainable ways to control pests in your yard. The first thing you need to realize is that pest control is all about control, not elimination. A few aphids won't kill you or your plants. One really useful method is companion planting. By keeping some only semi-desirable plants around your prize specimens, bugs will be less likely to choose your plant for a meal. Also, some plants deter pests altogether aphids don't like nasturtiums for example.
There are also specific pest control methods, such as putting out a saucer of beer to attract and drown slugs. A good article for getting rid of aphids is "Eco-Friendly Aphid Control". For an extensive and extremely thorough guide of organic solutions for any and all garden pests, see the Extremely Green Gardening Company's Organic Pest Control Guide.
Creating and learning to create a sustainable yard is an ongoing process, so any one article is only a place to start. Do you homework and get started on the road toward a landscape that serves your needs and the needs of future generations.