I agree with Arraxxon. I never use the method suggested by Wabe. It's easy but to my eyes, it doesn't look natural enough. Furthermore, in certain situations it can introduce noise.

There are many other methods to achieve ligher shadows. From top of mind, I remember these below. Each one of them fits a specific situation better than others, so make sure you experiment in your particular case. For example, increasing glow intensity (option d) can be wonderful in dark forest scenes, it creates a kind of mystical glow.

a) increase sky dome lighting gain
b) use radiosity (with or without increasing gain)
c) move light balance slider more towards ambient
d) increase glow intensity (in the 'sky, fog, haze' tab) and then increase haze/fog
e) use secondary lights
f) use environment map