It seems that everyone is aware that gardens can require some effort to maintain. Professionals in the landscape design and construction industry understand that the success of any outdoor space depends on thoughtful design, quality installation, and ongoing, intelligent maintenance. Not one of these three items can deliver the desired result without the other two. Nearly every client I've ever known has brought "low maintenance" up as part of their wish list, often right at the beginning of the first meeting. Let's consider:Thoughtful Design - the design process should take into account the needs and life-cycles of both living plant materials and non-living hardscape materials. Naturally derived hardscape materials may not be "alive", but they do change over time. Metals oxidize, woods weather, and stone materials can do both...Quality Installation - poor installation can cause good design ideas to fail miserably. Bad planting practices will kill your plants and trees, poorly installed paving will sink and shift, retaining walls can fail, and irrigation systems can be mal-adjusted in so many ways.Ongoing, Intelligent Maintenance - taking care of your investment is critical. There's no way to design around bad gardening practices or neglect. Understanding your plant and hardscape materials is half the battle to having a garden that doesn't require more of your (or your gardener's) time than is reasonable. Understanding is the key to this. For example, I firmly believe that you could grow roses (considered by many to be high maintenance) without much fuss if roses that are suitable for your area have been selected and you understand how to care for them.So if we keep the focus on maintenance, what are we talking about?

Pruning, shearing, dead-heading, weeding and mowing.
Fertilizing, spraying against disease or insects, watering.
Cleaning (water features, stains on hardscape), sealing (deck wood, stone, etc), repairing broken items, etc.

There are countless books and internet resources on low maintenance gardening. Not one of them can take the place of using good common sense. Below are a few quick thoughts:Containers:

They must be big enough to support the plants grown in them.
Smaller containers dry out faster, and don't have as much room for roots as larger containers.
Some plants are more tolerant of container culture than others. Shallow planters are better for some things than deep planters and vice versa.
Containers need a hole for drainage.
Dark containers heat up more and can cook the plants' root system (and dry out fastest).
Containers (or raised beds) of different materials create different environments for your plants: metal, wood, ceramic, terracotta, and plastic all have unique qualities that should be considered.

Plants:

Each different kind of plant has different needs.
Individual plants of the same kind are individuals and may not look exactly the same as the same kind of plant nearby. Healthy plants are better looking than plants that are struggling.
Lots of different plants are harder to care for than a simpler plant palette.
Plants grow. Trying to keep a naturally large tree small to fit a small space is ... less clever than planting a smaller growing tree in the first place.
Lawns are a lot of work, period. They don't have to be so bad, but the appearance that people typically demand of their lawns requires work and chemicals.
Plants have annual life cycles. Some go dormant, others look pretty much the same year-round. For example, Bulbs need their fading foliage to store up energy for the next season, and lavender starts to look ratty after about two years without annual pruning. Honor your plant's needs. Some plants re-seed themselves, others don't. How does that fit with your design intent?

Hardscaping:

Stone is a natural material and is meant to oxidize and weather naturally. I wholeheartedly disagree with sealants on stone to "keep it looking new". My feeling is that this is like applying clear nail polish, and that the true beauty of stone is expressed with age and patina. Choose surface finishes carefully for the out of doors as smooth stone can be slippery when wet.
Tiles 1/4" thick are not meant for paving. Materials this thin are usually meant for vertical applications or indoors.
Wood is a natural material that weathers and rots. My personal preference is not to use sealants on wood decks unless you really enjoy sanding them down and re-sealing them annually. Containers and other sources of consistent moisture will accelerate the decomposition of the wood on your deck, so be careful. Different woods degrade in different ways and at different rates. They also have very different costs associated with them. The most vulnerable part of lumber is the end cut. Working with the characteristics of wood can produce wonderful results, but ignoring its natural tendencies is fool-hearty.
Bricks are a man-made construct from natural materials. They can age beautifully like a soft stone or be chosen for their resistance to weathering.

Consider that Filoli in Woodside, CA has 12oo volunteers plus a paid staff to maintain it. I think it is totally worth it for such an impressive estate and extensive gardens. Oh, and make no mistake - those people know what they're doing, too. So what is low enough maintenance for you? One hour a week? Three? Do you need to hire someone, or are you going to do everything yourself, and do you really enjoy doing it, or will the work ruin your enjoyment of your outdoor space?Truly high maintenance: bonsai, String Garden, topiary, zonal denial (growing stuff that isn't really suited to where your garden is), and trying to cram in too many amenities so that the feel of the space suffers.Please add your thoughts in the comments!