For hot temperature, just follow the datasheet.
For low temperature, I remember someone last year trying to overclock a UNO with liquid nitrogen so I guess you won't ever meet any problems with low temps :-)
In his blog, that person shows that he could run his UNO at 65 MHz by reducing temperature to -196°C.
Of course, the process was more complex than just reducing the temperature and checking what happens: a lot of improvements were performed on the board.
The blog explains very well how various components can react to cryogenic temperatures; the major issues seemed to be capacitors which capacitance dramatically reduces under low temperatures.