Solutions for cooling solar panels
The answer depends on quantity.  The more complex the system, the more potential modes of failure.  The article mentioned by @Aryan Firouzian goes into great detail on several of these methods, but as a general rule, in order of increasing complexity:

Passive air cooling- heat exchangers on the back of the panel transfer heat to ambient air, airflow is provided by local air currents or convection.  There is no electricity cost to this method.
Active air cooling- same as above, but some of the energy from the panels is used to power fans.  I would imagine that other than in a few DIY arrangements, as soon as the complexity of fans are added, it becomes better to switch to water cooling.
Passive water cooling- Water circulates between panels and a radiator/evaporative cooler due to convection.
Active water cooling- Open loop- Water is pumped or sprayed over the surface of the panels.  This can be used to help with dust issues as well.  Does not necessarily require a radiator.  Water evaporating from the surface of the panels can transfer more heat away that the water could carry away with its thermal capacity alone.
Active liquid cooling- Closed loop- In it's simplest form, water is pumped through a heatsink on the back of the panels, through a radiator and back to the panels.  Optionally, the heat from the water can be used directly, harvested, and a system can also be built without photoelectronics being involved at all.

The increase in output is quite substantial, and can justify a fairly complex system if enough wattage is present.