To increase the pressure you have to do work on the system, thereby increasing the energy. To decrease the pressure (aslong as it is above 1 atm) you have to allow the system to do work. This a bit of a rough and ready explanation I'll admit.

Temperature is the mean kinetic energy of the particles in that system. The average kinetic energy of a system can said to be
[tex] E_k = \frac{3}{2}kT [/tex]
The ideal gas law links, pressure volume and temperature:
[tex]pV = nRT = NkT [/tex]
[itex]n[/itex] is number of moles; [itex]R[/itex]is molar gas constant; [itex]N[/itex] is number of molecules/particles; [itex]k[/itex] is the Boltzmann constant [itex]= \frac{R}{N_A}[/itex] where [itex]N_A[/itex] is Avagadro's constant.