What is the best way to make a speaker louder
Buy a new one, that has a better efficiency (in amateurish: higher dB value). This is not a bigger is better scenario. Speakers are delicate devices, needing precious balance to provide the quality, not just the power...

What specifies how loud the speaker is going to be? The main point is: louder means more air moved


Efficiency of conversion of electrical power to mechanical displacement

magnetic gap: the more precise the better
magnetic field density
coil quality and losses

DC resistance
capacitive and inductive qualities


mechanical properties

mass of cone and moving parts
flexibility of seam at the cone's perimeter
flexibility of cone (aka cone ripple, this varies with frequency)
cone area
cone shape
maximal cone displacement

mechanical properties of the surroundings

boxed speaker vs open air vs infinite wall vs bassreflex, bandpass and other enclosing a (cone feedback)
in theory, even the variations in air pressure and temperature... 



Also volume is one thing. Balanced acoustical properties is a completely different one. You can make a super loud speaker at a given frequency - that is practically not able to produce any other frequency than that. 

Actually, the mechanical limitations of the speakers lead to designing the 2, 3, and other multiple way systems. That is not just putting together speakers and hoping for the best: it is a science in its own.

Taking these into count, there is one thing left out to make a speaker louder, but still useful for something: designing a box, that emphasizes the right frequency band. By using  the Thiele-Small parameters, it is possible to do so, and there are quite some design programs helping designing a box for certain applications - mostly enhanced bass. However, this is not an "enter values - magic happens - build box - instant goodnes" - sometimes boxes that are good on paper are horrible in life...