When a plant has wilted, it is not hinting that it needs a drink; it is dying. Emergency resuscitation might be possible if you water immediately, but no plant will flourish if it is subjected to a regime of being watered and then left to become dehydrated. Nutrients are taken in with the water, and depriving a plant of food, even for a short time, damages its growth and its general health.
About nine-tenths of the weight of a herbaceous plant is water, and on a hot day, the average plant could easily absorb a couple of litres - a terrifying thought, even if your garden has an efficient watering system. It is water that keeps plants upright. So unless you want to replace your plants with plastic replicas, you cannot avoid the task of watering.
There are many old gardeners' tales about when and when not to water: should you do it in the early morning, or in the evening after the sun has gone down? Since watering is a way of helping nature out with what it does naturally (rain, that is) it doesn't much matter when you do it, since showers happen at any time. But if you water in the morning, avoid splashing the leaves, or they could be scorched by the early sun.
The point of watering is to provide enough of a reserve within the soil to keep the plants going until you can water again, and the secret is not in when, but how you water. The soil needs a thorough soaking, so that the water can penetrate deeply, and will be held in the ground to be absorbed gradually by the plants; better to do this every few days, allowing reserves to build up, than to apply water little but often. Hoses and watering cans should always be fitted with a rose or sprinkler to avoid sudden blasts of water damaging the plants; and avoid pools of water collecting on the soil surface, as they will evaporate before they can soak into the soil.
A good way to avoid too much evaporation is to water thoroughly and then apply a mulch, so that the soil is protected from the heat of the sun. Grass cuttings, mushroom compost and bark are all suitable, and black plastic, with holes cut in it for the plants to poke through, can also be used - not attractive enough in flower beds, perhaps, but very effective with fruit and vegetables. The other way to avoid water loss is to weed regularly: weeds are plants, too, needing water to survive, so if you leave them in the soil they will be depriving other plants of moisture.
In an attempt to cut down on the amount of time I spend watering, I was tempted at Chelsea this year to buy some terracotta cones. The idea is to push them into the ground until they are level with the soil, and keep them filled up with water, which will flow gradually downwards. Pieces of terracotta always look good in a garden, but this is only a more elegant (expensive) variation on some advice I was given when I first grew tomatoes in my allotment. Put an empty 5in plant pot (plastic will do) next to each plant, and keep that filled up with water. Assuming the pot has drainage holes, this will ensure a regular trickle down into the soil, encouraging the plant to make a deeper root system, and allowing it to take up water from the water table.
Some plants are better adapted to lack of water than others. If you are planning a break in a hot climate this summer, look around you for gardening inspiration. Broom and lavender flourish around the Mediterranean because they have adapted to the climate, and can last longer without water. It may be too late for this summer, but if you are looking for something to fill in the gaps left by dehydrated plants, make things easier for yourself. Anything hairy, spiny, fleshy, grey or glossy should fit the bill. So plant plenty of rosemary and sedum if you want your garden to be healthy when you return from your holiday.