This one involves a bit of an up-front cost but will add up to huge savings in the long run. Insulation will keep your home warm in the winter but also cool in the summer, saving money on your electricity bill as well.
For extra points, think about double-glazing your windows. This can reduce heat loss or gain by 30% compared to single glazing, as well as reducing noise from outside.
The kind of hot water system you run can make a big difference to your gas bill. There are two main kinds - an instantaneous system and storage system. A storage system heats a large amount of hot water - usually between 135 and 170 litres - and keeps it hot ready for use throughout the day. An instantaneous system only heats the water as you use it, so the system doesn't waste gas on heating water that's not being used. Because of this, instantaneous systems are much cheaper to run and - even better - they won't run out halfway through your shower.
Even if you have made the switch to instantaneous, you can save money by lowering the temperature it heats to. If you don't mind lowering the temperature a few degrees, you can save in the long run.
While you're there, consider patching any leaks and insulating your pipework - this will save any wasted hot water or heat when the water is transported. If you're not sure how, contact a qualified plumber.