There are three methods of doing this.
1. Enchanting the tool.
The 'unbreaking' enchantment will make the occasional use of the tool not reduce durability, how effective this is is random, but I higher level enchantment will increase the chance of a 'free use'.
2. Repairing tools
If you craft two un-enchanted tools together you will get more durability out of the result. The equation for this is:
Tool 1 durability + Tool 2 durability + (Maximum durability/10) = Resulting durability
Be warned repairing an enchanted item removes the enchantment.
Using a tool with an anvil can repair tools, either use two of the same tool, or a tool with its components (e.g. Iron axe and iron) be sure to try swapping the items round too, as it may be cheaper xp wise. This also has the added bonus of combining the enchantments.
Side note: Contrary to what is said in the question, using the wrong tool does not cost extra durability, only using them as weapons does.