Nobody has yet mentioned the importance of making sure your house is fully insulated. There should be 12 inches of insulation in the loft and, if your house has been built since, roughly, 1920 the cavities should be filled. If you are over 70 or in receipt of a disability benefit this can be done for free. In fact I keep getting rung up to inquire about insulation in my house and to be told about the availability of grants. If you want to find who can help you ring your local Age UK(Age Concern as was).
Go round all windows and doors and check whether there are any drafts coming in and if so use the cheap stickybacked foam strip you can buy in DIY stores to seal the gaps
If your radiators are against an outside wall put reflective material behind them. Just strips of cooking foil with the highly reflective side facing the radiator is sufficient, just tape it to the wall at the top, but you can buy rolls of polystyrene-foam backed foil at DIY stores that is easier to handle and cut to fit, again secure with sticky tape.
Even if you have double glazing draw your curtains as soon as it gets dark and if you have radiators under the window tuck the curtains behind them or push them right onto the windowsill so no heat can escape behind them. Make sure your curtains are lined, if not with special thermal linings then ordinary linings, or even another pair of curtains. Charity shops are often a good source of wellmade lined curtains. Have door curtains as well as window curtains.
Make sure you have your central heating serviced every year, not just for safety but also efficiency. If you can afford it replace your existing boiler with a condensing boiler. My son replaced his 1984 boiler with one and his gas consumption fell by nearly 30%. Again inquire about whether you can get a grant to help pay for this.
I am surprised how high some people's monthly bills are. We live in a large 4 bedroomed detached house built in 1467, so not built with energy efficiency in mind, and last year our gas bill averaged just under £70 a month. We have undertaken all the measures suggested above plus a little DIY internal double glazing (we are Listed so cannot change our single glazed windows), replacing our boiler, using low energy lightbulbs and some wall insulation inside (no cavities to fill) and we have halved our gas consumption over 10 years.
Last but not least, buy an electric blanket, they are bliss.