If your (desktop) computer is retail made update the power supply. That way your CPU and other hardware get the right amount of power they need to work at full potential.
Find one that is 20 to 50Watts higher then the stock. 80+ plus certified (80%+ energy efficiency) power supplies would be prefered so theres no overload.
I don't know much about laptops but you could try getting a stronger battery, if thats possable.
You could also get an External Hard Drive for data backup. Some people wouldn't think one could speed up a system but they can.
The one I got sped up my start time and software load time.