Minecraft wiki entry for Time-saving_tips, associated text: Furnaces are very cheap to make, and it is much quicker to process large quantities of material in parallel, so don't build just one furnace, build lots, and use them all at once.
In the early stages of your world, putting down a furnace to smelt ores or cook food while you gather materials nearby can help save time.
When smelting items – assuming you use coal or charcoal as a fuel – always try to add items to the furnace in multiples of eight. This avoids wasting fuel. If you really need to smelt less than eight of an item, use a furnace fueled with wooden planks, sticks or saplings. Two saplings or sticks will smelt one item; two planks will smelt three.
You should always keep plenty of fuel in your furnaces so that you don't have to refill them every time you need to smelt something.
With hopper-fed furnaces, add a switch to deactivate the top hopper. That way you can queue up a great many different items and smelt them all at once, reducing the need to gather by everything 8 times.
If you made one, keep your first wooden pickaxe (you can damage as much as you want) so you can put it in a furnace, it'll cook one item.
If you are out of coal and there are no coal mines nearby, cook a log in the furnace (not as a coal substitute, but in the top box) you will get charcoal, which works exactly like coal.
While charcoal is a good way to make torches without coal and when created using wooden planks as fuel is the cheapest wood fuel source available, the additional time and hassle of creating the charcoal may not be worthwhile when just using wood planks as fuel is only around 15% less efficient and saves both smelting time as well as time spent tending the furnace
Later in the game, you may want to use blaze rods as fuel, a blaze farm will produce more than you will ever need and they burn 50% longer. This way you will only need to mine a bit of coal every now and then for torches - or just use charcoal.
Blast furnaces and smokers can be used to smelt ores and food 2 times faster than a normal furnace respectively, but keep in mind that they can't smelt cobblestone, sand, cactus, clay or other general things.