These methods of reducing electricity usage cost under £10 and will save more than they cost within a few months.
More commonly known as 'low energy bulbs', these use up to 80% less electricity than a normal incandesant lamp. They do cost more than normal lamps, typically between £1 and £5 each. However, used for a couple of hours a day, the additional cost will be recovered in less than a year. Anything after that is money saved, and CFLs should last for several years.
CFLs are not necessarily better for the environment, since the resources used to manufacture and dispose of them are far greater than a tungsten filament lamp. They also contain trace amounts of toxic chemicals.
LED lamps are even better, since these use even less power, give out virtually no heat, and should last for decades. Unfortunately, they are not yet suited to general lighting, as lamps which have a light output sufficient for a whole room are extremely expensive.
If you have one of those overpowered 500W security floodlights, remove the 500W lamp and replace it with a 300W version. Still more than bright enough for most purposes, and uses 40% less electricity.
You can now get fluorescent lamps to replace 300 or 500W halogen lamps. However, these replacement lamps may not work properly with some motion sensors, and in many cases it is cheaper to replace the whole fitting.
If you forget or just can't be bothered to switch everything off at the socket, an easy and cheap alternative is a 24 hour plug-in timer. These can be obtained for around £5 - less if purchased in multiple packs.
If you only watch television in the evening, set the timer to be 'on' between 4pm and 12 midnight, and 'off' at all other times. Plug the TV, DVD player etc. into the timer, and the power will be disconnected for 15 hours a day.
Is your airing cupboard nice and hot? If it is, then your hot water cylinder needs more insulation.
Insulation can be added by purchasing a 'cylinder jacket' - an overpriced collection of thin triangular panels, which are secured around the cylinder with string.
A far more effective option is to purchase a couple of rolls of 150mm loft insulation and wrap this around the cylinder. Use the type with a plastic covering to avoid excessive dust.
Whatever is used, make sure that any electrical cables are NOT covered by the insulation.