Here are a few practical ideas to help you save the fuel consumed by liquid and gas fuelled stoves when backpacking.
Let's start with a basic question. I know of some backpackers who just boil water for drinks/soups and eat high calorie pre-prepared food. They don't cook on backpacking trips! Some experienced backpackers don't even carry stoves/cooking equipment for short trips in summer. These people are avoiding the 'fuss' and paraphenalia of cooking, they don't do washing-up and *might* save pack weight. Personally, I like cooking outdoors. I've tried other approaches and they don't suit me - but it is a valid first question to ask yourself: to cook or not to cook?
Presuming the answer is "cook" then how much fuel you burn and so will need to carry on a backpacking / camping trip is influenced by a lot of things including: the cooking equipment, the type of food you cook, cooking method and environmental factors. We can split the advice into before the backpacking trip and during the backpacking trip itself.
Before your backpacking trip
Good, properly-maintained equipment
If you're looking to purchase a new stove and/or cooking pot remember that a balanced cooking stove and pot system will ensure that the flame from the stove is directed evenly to the base of the pot. It's a good idea to see a stove/pot combination working or to at least do some research where reviewers comment about how the two components work together. It is generally accepted that shorter and wider pots are more fuel efficient than taller, narrower pots. This is because more of the flame is directed to heating the pot base rather than excess flame travelling up the side of the pot. If flame is leaping up the side of the pot not only is this potentially dangerous but it wastes a lot of the fuels energy. Choose a pot with a good-fitting lid when it comes to saving fuel this is the Top Tip. Personally I like pots with a lid and pouring spout, like the MSR Titan Kettle (which also has justifiably massive reputation).
If you cook with meths and your pride and joy is a shiny Titanium or other pot, my advice is to allow the patina to gradually build-up on your pot. After each burn, wipe off the excess soot and gradually your pot will turn a matt black/brown. This is good! A blackened titanium cook pot may not look so pretty but it is more efficient at capturing the stove's energy. So if you're buying a Used pot on eBay - it might not sound so logical but don't necessarily scroll past a blackened pot and go for the bright shiny one! Hard-anodised aluminium pots conduct heat very efficiently, are relatively inexpensive compared to Titanium and wear pretty well. Pots with built-in heat exchangers on their bases are expensive and weigh more but they do work and are worth considering for long backpacking trips/expeditions and especially in cold weather.
Ensure your stove is properly-maintained and that any soot is cleaned from around the burner/jets. Check first for any blocked burner jets, then fire up the stove and check that the burn is even with the same size of flame around the burner. Once the stove is fully-working, check to see if there's a blue flame as this indicates that the fuel is being burnt efficiently. It's also worth doing these basic stove maintenance checks during the trip itself.
Don't forget your stove windshield. For meths and esbit stoves, clever pieces of kit like the Caldera Cone not only protect the flame from the wind, they can also help to use the energy from the stove more efficiently than vertical walled windshields. Be careful when shielding a gas canister from wind as excess heat must not be conducted to the canister itself (see your manufacturers Safety Advice).
A pot cosie (cozy) is an ingenious invention. Cosies are made from lightweight insulation that traps the heat from already hot food inside the cooking pot so that it continues to cook and stay hot once it is removed from the stove. Pot cosies are really efficient fuel savers for any food that needs to be simmered (porridge, pasta, rice etc.). They can also help cut down on condensation/steam, see more about cosies below.
Choice of food
Modern freeze dried food is very convenient and when properly rehydrated can often taste very good. Meals prepared by simply adding boiling water are inherently efficient, the meal has already been cooked, you are just rehydrating it. I use these quite a lot along with a insulative pouch that helps rehydrate the food efficiently.
Think about the type of food and how you're going to cook it. If you continually have to stir the food while it warms through you will not benefit from having the pot lid in place. Another option is to include meals that can combine ingredients that need/don't need to be cooked e.g. some pre-cooked protein (fish/meat) to accompany rice or couscous (minimal cooking).
During your backpacking trip
Where and when you cook can be a major factor, before you start cooking make sure you and your cooking system are protected from the elements. Even the lightest wind will blow the stove flame and guarantee a huge waste of the fuels' energy if the stove is not protected. Even with a windshield, in a strong wind a stoves' flame will tend to get displaced and shoot up the sides of the pot. One exception appears to be the Trangia which seems bombproof almost whatever the weather throws at you. If it is just a bit windy, try to find a sheltered place to cook using natural windbreaks like rocks. What if there is wind and rain? I'm not going to endorse or dismiss cooking in a tent porch. It's an option and whether or not you do it is your decision considering the risks. Just bear in mind not only the risk of fire but also Carbon Monoxide (CO) which is a killer. All stoves produce CO and you need adequate ventilation when burning fuel. Temperature and height also effect a stoves' efficiency. In the UK we don't need to worry too much about altitude effects on stove efficiency but if you cook with meths and it's a cold day it can help to pre-warm the meths bottle with your body heat and even pre-warm the meths stove itself in the same way - it's surprising how much better the meths will light and burn. Some winter campers take their gas canisters/fuel bottles to bed with them. Another winter tip is to make the fuel one of the last items you pack. Keep it for at least 24 hours before your trip at room temperature, then wrap the fuel container/canister in an insulation layer and/or inside a pot cosie. The heat won't last long once unpacked but even if it lasts only for your first meal it'll be warmer than the ambient temperature and the stove will light and burn more efficiently.
Efficient cooking process
Get everything you need to cook the meal close-to-hand before lighting the stove. A sure-fire way to burn excessive fuel is to get the stove going then start searching around for cooking utensils, ingredients etc. Make sure you boil the right amount of water and don't waste boiled cooking water. If it can be used again - depending on what you cooked in it - why not use it e.g. use the cooking water from a pasta dish to make up a cup of soup to accompany your meal. It's worth experimenting with this in advance, some foods can produce very starchy and/or salty water and might not be suitable for everyone.
You paid for it, use it. Your cooking pots' lid is probably the best fuel saver around! A tight fitting lid is a major bonus. Even if your pot didn't come with a lid a home-made lightweight foil lid made from thick baking foil (takeaway tray or aluminium pie dishes) will save fuel. Punch a couple of holes and use a paper clip or twist-tie to make a basic lid handle.
How much of a 'hurry' are you in to eat? Using a lower flame will take more time to bring the water/food to a boil but it will also use less fuel than the fast-and-furious approach. I generally boil up some water for a hot drink and then cook more leisurely. Consider using a pot cosie, these pieces of kit trap the heat in your pot so rather than consuming fuel to simmer, your food or water can stay hot. One of the best features of cosie is that food will continue to cook (simmer) inside the cosie so you can prepare your meal, get on with other jobs around camp and then come back to a piping hot, perfectly cooked meal. Google the term "pot cozy" to find out more - these simple pieces of kit are used extensively by North American backpackers. If you use 15-33% of fuel to maintain a simmer, imagine the amount of fuel wasted compared with turning off the stove and using a cosie.
Finally, don't let a pot of water boil unnecessarily, use the water straightaway or at least turn the stove down to simmer or use a pot cosie to conserve the heat.
So there's a few ideas to consider when you take your next backpacking trip. Enjoy!!