Over the last few years, the South-East part of the UK has suffered from water shortages and this is likely to continue with climate change and the increase in population. While drought is a specific level of water shortage (with standpipes etc.), hose pipe bans are not uncommon. Elsewhere in the UK, there may be no shortage of water or hose-pipe ban but planning to use less tap water is always a good idea.
During a period of water shortage, and especially if a hosepipe ban is in place, it makes sense to save any rain which falls on your house, shed or garage. Water butts can be bought or any suitable barrel or container can be used (you need to consider what was originally in the container as any residue may be harmful to your plants). Remember to raise the butt/barrel off the ground so that watering cans can be filled - stands for water butts are readily available, otherwise a paving slab supported by firm bricks or concrete blocks to the required height will suffice.
Rainwater divert kits are available which can easily be fitted to most plastic downpipes, most kits are universal to fit both round and square pipes. Alternatively a straight downpipe can be fitted into the top of the butt/barrel - the only problem then is to fit some form of overflow.
Increasing the amount of organic matter in the soil of a vegetable or flower bed will help it retain moisture - but this isn't something which can be done when the dry spell arrives, you have to do it in advance. In the case of heavy soils, it is a good idea to dig in farmyard manure in the autumn or winter before, while in the case of light soils, dig it in in spring to avoid the nutrients being washed out.
In heavy clay soils, organic matter will open up the structure while in light, sandy soils, it will add body and nutrients. In both cases root growth will also be encouraged with increased organic matter which will help the plants find what water there is and help them through period of stress.
Either compost or manure can be used. See this page for more information about organic manures.
Putting a mulch on top of moist soil will help keep it cool and reduce evaporation, a good thick mulch will also help suppress the weeds that will be competing with the plants for what water there is.
Various materials can be used as a mulch; composted bark, ordinary garden compost, grass cuttings, even old carpets. Avoid using straw or woodchippings around plants as they tend to take nitrogen out of the soil. Also avoid any material which may contain weed seeds (such as farmyard manure) as its use may cause more problems than it solves.
Do not use peat as a mulch, such use is a waste of a valuable material and should it dry out, it is difficult to re-wet.
See this page for more information about mulching.
Various products (such as swell gel) are available which, when mixed with compost, will absorb several hundred times its own weight in moisture and slowly release it as it is needed.
The use of this type of product in pots, planters, containers and hanging baskets will reduce the amount of water lost and decrease the amount of watering required.
Avoid watering your plants during the heat of the day, watering either in the early morning or in the cool of the evening will reduce the amount of water lost through evaporation.
When you do water, give the plants a good thorough watering at their base, this will allow the water to penetrate deep into the soil and help the deep roots develop rather than just the surface roots.
Only water plants when they need it, by giving them a thorough watering you may find that you need do it less frequently.
Watering into plastic bottles inserted up-side-down with their bases removed next to plants will help the water reach the roots and reduce the amount of water lost through evaporation.
Plants in containers will require watering more frequently than those in the ground, and small containers more frequently (although with less water) than large containers.
Positioning any containers nearer to the tap will reduce the amount of work needed each time they are watered.
Established lawns should be able to survive long periods without water, they may loss their green appearance but will recover quickly once there is some rain.
Set the cutting height higher than normal during dry weather and cut the lawn less frequent.