A fuel tank will have a small quantity of fuel remaining in it when indicating empty, this is known as unusable fuel. It's best to avoid drawing every last drop as the bottom often has contaminants such as rust, water and sediment which you don't want clogging up the system. Carrying a sensible reserve and avoiding getting into a low fuel situation in the first place is the best option.
In flight it's possible to change what’s known as the cost index which flys a profile based on the airlines requirements. Basically you can fly minimum time, minimum fuel or somewhere in between. You can trade off a later arrival for more remaining fuel if reserves are getting a bit tight but you need to do this well ahead so the change has time to work.