I've loads of tips for conserving water in the garden, but they're scattered all over the internet and various magazines. So I'm creating this post to bring them all together; if I discover more I'll add them here.
Reducing the need for water
While the soil is damp, add a mulch. This can be organic e.g. leafmould, compost, bark chippings (below left) or inorganic e.g. gravel (below right), slate chippings. This will help prevent water loss by evapouration.
Avoid bare soil. Cover soil with mulch (as above) or drought resistant ground-cover plants.
Minimise your use of annual bedding plants and vegetables. These do not live long enough to grow deep roots so need frequent watering. Perennials are better, shrubs better still.
Seek out drought-resistant plants. Native plants which grow locally are also a good idea; they are naturally adapted to your area.
Allow your grass to grow longer.
Only water if it's really needed. Mature trees and shrubs are unlikely to need any watering. Most perennials and younger shrubs only need watering once a week. Containers, annuals and thirsty vegetables (such as tomatoes, cucumbers, courgettes) may need to be watered every 1-2 days. Mature lawns don't need watering: they'll go brown but they will recover.
Water the soil not the plant: Watering the leaves or flowers of a plant risks scorch if the water evapourates off them, or mildew if it sits in place. Aim to water the soil around the base of the plant rather than the plant itself.
Water in the early morning or evening: watering in the middle of the day is likely to result in a lot of it evapourating before it had a chance to penetrate the soil properly.
Water heavily but infrequently: this means the water will penetrate to a greater depth, which in turn encourages the plant to grow deeper roots. With trees and shrubs, over time the roots will become deep enough to reach water even if it's well below the surface, so they will barely ever need watering once mature.
Using a watering can means lots of walking back and forth and may help reduce the temptation to over-water. If you use a hose get a nozzle which can easily be turned on and off, and turn it off when you between plants.
Don't use a sprinkler, it will water everything around it indiscriminately and much of the water may be lost to evapouration, wind spray or landing on hard landscaping. A drip or seepage system is better if you really need to find an automated solution, but it's best to use it with a rain sensor so that it doesn't water if it doesn't need to. Be aware that most local authorities require you to have a water metre fitted if you are going to use any form of automatic watering system.
Help the water get down to the roots
Water may simply sit on the surface, or run off to lower levels away from your plants. Here are some ways to help it penetrate the soil and reach the roots:
Creating a small ridge around the plant so that there is a bowl-shape in the middle for the water to settle in prevents run-off. Alternatively a shallow moat around the base of the plant or a channel alongside a row of vegetables, both of which do the same thing.
Sink a water bottle, plant pots with holes in it or short length of pipe into the soil next to the plant, then water into this. It's best to do this when you first put the plant in position or you'll risk disturbing the roots.
Wet the surface and come back 10 mins later to water more thoroughly. The initial wetting will break the surface and make the soil ready to absorb more water.
Use luke-warm water or add a few drops of hand wash or washing-up liquid and mix it in. This reduces the surface tension of the water, so that droplets are less likely to pool on the surface of the soil.
Avoid using the tap
There are various ways in which you can collect and re-use water. A lot of plants prefer rain water, but if this is in short supply you can also use "grey" water e.g. from baths, showers or washing-up.
Put water butts wherever you can. Lots of rain falls onto the roofs of our houses and greenhouses then makes its way via the gutter into the downpipe. If you have the space you can add water butts to each of these downpipes. I've heard that very large water butts may buckle when full, but you can use slightly smaller ones and join them together to increase capacity.
Remember you can even put a water butt on the downpipe from your shower or bath. If the soap is safe to wash yourself in it's fine for plants too, and they won't mind a bit of dirt. Personally I avoid using this water on vegetables, I don't think there's any real reason not to but I find it a bit off-putting, but it's fine for the rest of the garden.
If you're having a new patio or deck fitted, consider getting a water tank fitted underneath it and channel your rain water into that.
Some sources advise collecting rainwater in any container you have available e.g. buckets, pans, old (sealed) sinks. The problem I've encountered with this is that after a few days the water tends to attract mosquitoes who lay their eggs in it, so I prefer to stick to covered butts.
If you like to run your shower for a minute or two until it runs warm, collect this water in a bucket and use it for the garden. If you wash up by hand, you can use the water left over in the bowl. Don't use water from a dishwasher however as the salt and other chemicals can harm plants.
Plants in containers
Use larger containers if possible. These hold more soil and retain water for longer.
Mix water retaining granules or gel (see picture below) into the compost at the time of planting.
Use saucers to collect rainwater but don't leave plants sitting in these for long periods of time or you may rot the roots. It's better to add the water collected to your water butt, or pour it onto dry plants.
Avoid using terracotta. It's beautiful but also absorbent, so plants in terracotta pots will need more water as much of it will soak through the pot.