If you're talking about DJ mixing rather than production mixing, check out one of the sticky threads in the DJ booth - it has a list of keys for tunes. Best example is - pick 2 tunes on the list in the same key, then try mixing them - it'll sound superb
As you understand key mixing more you'll be able to choose different keys to mix into to get different effects (ie to provide a bit more energy in the mix, or to slow things down a bit)