Watering

Maximize the water-holding capacity of the soil by adding lots of well rotted organic matter on an annual basis
Eliminate the competition for water by keeping weeds down.
Mulch after a good rainfall.
Conserve water by making a shallow moat or reservoir around plants.
Collect as much water as you can in water butts. If you have a shed, greenhouse, lean to etc, make guttering and down pipes to collect it from the roof and stop them from clogging with debris. Keep your butts covered to prevent evaporation.
If your water butts are over flowing, empty some water to a spare container.
Keep checking the weather forecast. Don't water before rain fall is expected.
Don't make drainage ditches, you'll need the moisture later and you are leaching valuable nutrients away.
Sink a flower pot or section of pipe into the ground to get water down to the roots of thirsty plants, like courgettes or cucumbers.
Water in the cool of the evening to avoid rapid evaporation though it is better to water in the morning before sunrise.
We all know rain-water is superior to tap water for plants and it is good for the environment, saving energy and costing nothing.
Do not fill your water butts/containers from the mains water tanks. Not only is this wasteful, but you are depriving other gardeners of a shared resource.

It is best to dribble water into the seed drill before sowing.
Watering overhead can make the soil 'cap', or form a crust, making it difficult for the seeds to emerge.
If you need to water after the seedlings emerge, use a fine rose on your watering can.

Transplants are particularly vulnerable and need nursing until they establish. Water before and after transplanting. This is the one occasion when you should water little but often.
You can manipulate the plants performance by timing and the amount of water you give them. The general response to watering is leaf growth, which is exactly what you want for leafy vegetables. However it is not what you want for peas and beans. It is better to keep them on the dry side until they flower. At this point the root activity slows down and the plant could do with a boost. A good watering at this stage will produce more flowers, hence more peas and beans.

As a guide, all plants need sufficient water when young until their roots are established.
Vegetables grown for their leaves should never be allowed to dry out.
Fast-growing plants, like courgettes and marrows, need plenty of water throughout their short growing season.
Over watering can produce a lot of soft growth attractive to pests and make plants susceptible to cold or rotting off.
Edibles grown for their roots, pods and fruits need steady but not excessive water until the flowers, fruits or roots start to form.
Potatoes profit from a good dousing when the tubers are the size of marbles.
When watering, the water should soak the lower depths of the soil to encourage the roots to grow down. If you just sprinkle water on the top, the roots will look no further and can dry out very quickly.
Water Brassicas at the base to avoid the damp conditions on the leaves that can encourage fungal disease.
For plants sensitive to cold (tomatoes & aubergines), use water from your water butt.
Check the moisture content of the soil by digging a hole with a trowel. If it is damp 9-12 inches deep then the roots of established plants will have access to moisture. This doesn't apply to seedlings, obviously.